January 2019 issue of Nightfall, a quarterly journal about astronomy in South Africa. by Douglas Bullis

N I G H T FA L L JOURNAL OF THE DEEP-SKY SECTION ASTRONOMICAL SOCIETY OF SOUTHERN AFRICA

Source: ESO/H Dras et al. 2016

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Vol. 3 No. 1 January 2019

N IGHTFALL Astronomical Society of Southern Africa Vol. 3 ISSUE #1 January 2018

Editor-in-Chief

Douglas Bullis

Editor

Auke Slotegraaf

Contributing Writer

Magda Streicher

Design, Layout, Production

Dana De Zoysa

Published October 2018 by the Astronomical Society of Southern Africa. Observatory, Cape Town, South Africa Editorial address: https://assa.saao.ac.za/ Website: http://assa.saao.ac.za/sections/deep-sky/nightfall/ Print-on-demand editions produced by Atelier Books LLC Postnet 18, Private Bag X1672 Grahamstown 6140, South Africa email: assa.nightfall@gmail.com Individual articles and images Copyright © 2019 by their respective creators.

ISSN 2617-7331

Star plot of the Pleiades from Galileo’s Sidereus Nuncius, 1610. Courtesy of the Galileo Musem, Florence, Italy.

Image: Comet Wirtanen November 2018 by Martin Heigan

IN THIS ISSUE 4 Magda's Journey to Iceland 14 Simeis 147 Supernova Remnant by Martin Heigan 29 Jaco Brink's Mission Impossible 34 Why are Comets so Green? 40 Johan Moolman, Turning the Pages of the Sky 52 Dirk Rossouwâ&#x20AC;&#x2122;s Melkbosch Skies Observatory 60 A Dialogue between Two World Systems â&#x20AC;&#x201C; Early Celestial Cartography 70 Mr. Herschel Does His Homework 72 The Gamma Velorum Cluster 79 Andrew James History of the Great Orion Nebula 80 The Man Who Discovered the Orion Nebula 90 The Horsehead Flings its Mane

Magda Streicher ˝

Source:

Journey into the cold M

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When Iceland obtained its independence from Denmark, the spoken and written language was one of the most complex to be found in any country â&#x20AC;&#x201D; almost impossible for foreigners to pronounce and write. This clean, unspoilt, country makes use of tts natural volcanic activity to supply hot water and power to every home. Summer produces a magical green landscape due to the rich volcanic soil, enough to provision for animal feed to be stored for the long winter. Fruit and vegetables are cultivated successfully in tunnels with the aid of new technology.

Sometimes when you don’t get what you want, you get what you need. Some events in life remain with us forever, while others disappear with time. What is beyond doubt, is that a visit to the extraordinary country of Iceland would be difficult to replace and impossible to forget – the memories would never fade. “Excited” would be understating how I felt at the prospect of visiting that multifaceted country. Knowing that we could expect icy cold weather I packed everything that would effectively protect me against the cold – or so I thought! But freezing aside, there was a huge excitement in me and full confidence that I would get to see the Aurora Borealis (northern lights). This natural wonder seen against a dark night sky must surely be the purest and cleanest form of miraging, dancing with paint strokes of bright colours. The best and rarest phenomena in the night skies are not always freely available and frequently observable – think of solar eclipses and the Aurora Borealis or Aurora Australis as examples. The polar circles are where the hot particles of the sun collide with the magnetic fields to create and display the latter two spectacular phenomena.

I have never been so cold in my life, but the great expectation somehow served to calm that cold and make it more bearable. Iceland was in the throes of winter, and the extensive white landscape was covered with snow and ice. The bus offered wonderful warmth – until we had to alight at the Grabok Crater. The beauty was overwhelming, and the sight of the flowing lava far exceeded our expectations. In the distance a majestic crater could be seen which was dormant at the time, but which, as we were told, was capable of a mighty eruption at any time. Iceland has the greatest number of volcanoes, waterfalls, glaciers and geysers because of its particular location on the earth’s crust. The impressive Atjaflatjayoka glacier, the largest of its kind in Europe, forces its frozen way through mountain peaks to pit itself against the earth’s forces.

Grabok Crater 6

We journeyed through hot springs terrain (97 Celsius), travelled through snow storms, pondering the unique country and exceptional countryside. Every evening we braved the cold, below-freezing night air in search of the elusive Aurora Borealis. But the weather remained overcast and we learned that this has been one of the coldest weeks yet during Icelandâ&#x20AC;&#x2122;s winter this year.

Probably the best-known of the many volcanoes in Iceland is Hatla, which in 2010 showed what the seething magma of molten rock deep in the earth is capable of in terms of blasting its way up into the sky. Our excellent Icelandic tour guide had many stories to share about survival in and adaptation to the conditions of this multifaceted, wonderful country.

Possibly one of the most impressive parts of Iceland is the Haukadalur area with its lava fields, where steam bubbles from the earthâ&#x20AC;&#x2122;s crust. The active geysers, whirlpools of boiling water, reach maximum pressure and then almost literally explode, shooting their steam many metres up into the air. It makes for a sense of vulnerability to realise that below the surface there is this constant boiling and churning of lava, hot water and steam, all accompanied by a strong smell of sulphur. These unforgettable, captivating impressions and thoughts are further heightened by warm bread, Icelandic butter, geothermally boiled eggs, herring and Geysir Schnapps for the cold.

One of the most beautiful waterfalls is the Hraunfossar, which consists of a series of falls dropping over a lava ridge. Light snow was falling while we were there, and it was a fairy tale scene that etched itself on to this South African ladyâ&#x20AC;&#x2122;s memory. Some parts of the waterfall are already frozen. The light snow drifted down on to the plants, and the white snowflakes forming a thin veil in front of my face produced unspoilt beauty. Lava fields from which steam escapes tell the story of smouldering hot activity below the crust of the earth.

The Vatnajรถkull Park is the largest in Europe at about 12 000 square kilometres. The enchanting, incredible, white snowy landscape is infinite, the beauty overwhelming. The crackling of crispy snow and ice underfoot added to this once-in-alifetime experience which will forever remain etched in my memory. 10

The Skaftafell National Park is where the true beauty of Iceland can be seen, with the Jökulsàrlòn glacier with its floating icebergs and black sandy beach, aptly named the Diamond Beach with its beautiful ice fragments appearing like glistening jewels on the black velvety sand. Icy crystals sparkle in the sun like diamonds, and in the distance the snow resembles whipped cream topped with grated dark chocolate for miles on end. The beauty and splendour of the lagoon are indescribable, a wonderful image forever captured in the memory.

But finally, on our second to last night in Iceland, the bright starlight broke through and the Aurora Borealis appeared as a bright green band against the dark night sky. Stars shone through the flimsy veil stretching from north to east in a graceful band. Here and there a hazy kink appeared and it looked as though the blowing steam was playing a concertina, shimmering and dancing, almost ebbing and flowing in glowing, graceful red and purple lines painting the sky.

Simeis 147 by Martin Heigan

Martin Heiganâ&#x20AC;&#x2122;s vivid portrait of the complex supernova remnant Simeis 147 begs two questions: What is it? How did he do it?

Imaging the Simeis 147 Supernova Remnant Martin Heigan

The Nitrogen in our DNA, the Calcium in our teeth, the Iron in our blood, the Carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff. - Carl Sagan, Cosmos

Technical Info

Imaging Simeis 147 was one of the most challenging — and thus rewarding — objects I have ever attempted. Technically it is a wide-field Narrowband HOO Palette mosaic (R=Hα, G=OIII, B=OIII) of the faint supernova remnant Simeis 147. This delicate tracery of near-nothingness halfway between us and the Perseus Arm of our Galaxy is also known as the Spaghetti Nebula, Sharpless 2-240 or SNR G180.0-01.7. The nebulous area is very large, about 3° in the long dimension, with an irregular cumulus-like shell of filamentary dust and gas. The remnant is about 3,000 light-years away and is approximately 40,000 years old. After the star exploded, a rapidly spinning neutron star (Pulsar PSR J0538+2817), was left behind in the Nebula core, emitting a powerful radio and even more powerful X-ray signal.

• 4 panel wide-field mosaic. • 16 x 600 sec. 7nm hydrogen-alpha (Hα) per panel. • 16 x 600 sec. 6.5nm doubly Ionized oxygen (OIII) per panel. • William Optics Star 71mm f/4.9 Imaging APO Refractor. • Sensor cooled to -15°C on my QHY163M. • Calibration frames: bias, darks and flats. • SGP Mosaic and Framing Wizard. • Astrometry.net ANSVR Solver via SGP. • Pre-Processing and linear workflow in PixInsight, finished in Photoshop.

Astrometry Info

About this image This image is the result of photographing during the 2018 Christmas/New Year festive season in the Southern Hemisphere, over several nights as the weather permitted, in locations and ambient light conditions ranging from my Telescope Pier at home to dark rural skies. Deep Sky Objects like this push the limits of my modest telescope gear, especially my mount. It is rewarding when you have to work hard for an image, and wide-field mosaics are a challenge redoubled. Simeis 147 in Taurus/Auriga is a good example of how difficult very faint but large wide-field Deep Sky Objects can be, and yet how breathtakingly beautiful they are when everything comes out right.

• Center RA, Deg: 85.239, 27.931 • Center RA, hms: 05h 40m 57.427s • Center Dec, dms: +27° 55' 52.133" • Size: 4.76 x 3.65 deg • Radius: 2.998 deg • Pixel scale: 10.7 arcsec/pixel • Orientation: Up is 263 degrees E of N • View an Annotated Sky Chart for this image. • View this image in the WorldWideTelescope.

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Balancing the data/palette mix is where the artist takes a bow.

The elements in a Deep Sky Object dictate which Narrowband wavelengths will give the best results. As there is not much happening in the green part of the spectrum in this type of object, Hydrogen-Alpha and Oxygen III is a good choice for SNR bi-colour Astrophotography. Narrowband also makes it possible to image in light polluted skies, as it blocks everything except the desired wavelengths of light. Imaging a vast multi-panel mosaic with only 2 Narrowband filters, also saves a lot of time. HÎą falls in the red part of the spectrum, and OIII in the blue part of the spectrum. I didn't want to mess with that in the processing, so I chose to use the HOO Palette. The OIII blue channel is simply mapped to the green channel as well, leaving the red and blue colours unchanged. I like to call this enhanced colour, as the colour is not false but only boosted due to the amount integration time.

Spaghetti in the sky Dana De Zoysa & Doug Bullis

Simeis 147 is colloquially known as the Spaghetti Nebula. In loftier circles it is SNR G180.0-01.7, Shajn 147, or Sharpless 2-240. In HII the sauce looks a bit rich on the red sauce. In Hα the noodles are soap bubbles in the sunset. In OIII pearls on a black dress. Sim 147 is a supernova remnant (SNR) directly astern of the Galactic anticentre as the sun rudders into the Sagittarian Sea. Viewed from the disc plane (our perspective) it is barely two degrees south of the centreline. Viewed from above it straddles Auriga and Taurus very close to the Galactic S polar axis. It does not appear on many star maps 20 years or older because it is one of most difficult extended objects to spot and has not been much of an amateur target till the last decade. Only four small pockets rise above the 25 mags per sq. arcsec (25 MPSAS) threshold, the lowest luminance most human eyes can see. The images below show the approximate locations. How do we prepare for the search?

firmed” sighting in these conditions means three unquestionable glimpses repeated two times in a given night over three different nights. Suitable warmup candidates are the Ursa Minor Dwarf, the Draco Dwarf, Fornax and 4 of its globulars, Phoenix, Sculptor, WLM, IC 1613, Leo X, Leo T, and for patient observers, the Pegasus Dwarf, and Andromeda VI. Since you’re in the area, Pal 2 in Auriga makes a nice side-trip, but faint GCs aren’t as helpful as dwarfs because their round glow is easier to spot than an extended filament when using averted vision. (You can always use poor luminosity threshold as an excuse to buy a larger scope.)

The big picture Simeis 147 is a middle-age supernova remnant ≈40,000 years old. Some of S147’s filaments have a different chemical composition on one side than filaments on the opposite side. That’s a clue to its origin. There is also a velocity gradient favouring the upper-left filaments but not the lower right. A velocity gradient in an extended thin nebula point to angular momen-tum left over after a binary decoupling.

Warmer-uppers The best preparation a hobbyist can make to tackle S147 is to log a lot of dwarf galaxies in the observations book. Dwarfs share Sim 147’s qualities of very faint threshold, luminosity density gradient, and starthick background confusion. A “ConPlot courtesy of Rich Jakiel in astronomy-mall.com.

Image adapted from Thomas W Earle, 2007.

Lopsided gradients are hard to explain because of their large surface areas and low gas densities. Clue #2. Aladin Lite multiband images show that the old supernova remnant PSR J0538+2817, visible mainly in XMM Newton X-ray, is also located well off-centre in the direction of the most rapid filament expansion. This suggests bubble breakout in which the expansion velocity of a gas parcel exceeds the binding energy of its bubble surface and breaks out into champagne flow. The Gordian Knot solution to these clues is that Sim 147’s original progenitor star was a part of binary O system whose components Bubble breakout in CBT1 Abell 85 were so close to each other that they exchanged material via contact Roche Lobes. Type O similar-mass binaries closer than 1 AU (the Earth-Sun distance) are close enough that they exchange surface ejecta gases via their Roche lobes. When one of the pair goes supernova, the drastic and sudden loss of mass on one side releases the second star to wander off as an O runaway star. The runaway’s new transverse velocity is its original orbital velocity minus the momentum transfer losses donated to the overall association as the runaway departs. Moreover, the shock wave from a binary supernova blasts across the surviving O star, stripping material from the star’s surface and shallower envelope layers. That would account for Sim 147’s metallicity anomaly. Roche lobe gas would be more hydrogen-rich while envelope gas would be more helium-rich. Excess helium in a remnant’s spectra implies envelope stripping. Calculations show that predicted metallicities in models approximate what real metallicities are in Sim 147. With these clues in hand, astronomers went O

(±350) light-years distant — about half again further out toward the Perseus arm than the young open star clusters M35-36-37-38 in Auriga and Gemini. That puts it in the interarm region between the Orion Spur and Perseus Arm. The progenitor star’s explosion left a 9 RPM neutron star packed so densely that it is 2.3 times the density of an atomic nucleus. It is seething in magnetic fields so powerful they’d tear off your belt buckle around Pluto. Heaven help you if you have those old mercury-amalgam tooth fillings. Not for nothing are pulsars, black holes, and magnetars called “exotics” in the astrophysics literature. Sim 147 is a roughly oblate spherical shell dense with pendulous filamentary loops. The leading edges glow because they are expanding

In a binary system, a pair of stars close enough to be inside their Roche limit become gravitationally unstable. The instability initially affects surface layers, which detach from the stars and fall into a Roche lobe. Mass transfer occurs in two of the three situations where a Roche lobe exists. In a simple detached binary there is no exchange between the stars because the Roche lobes do not fill with gas. In semi-detached binary one star fills its Roche lobe and transfers mass to the denser star. In a contact binary both stars fill their Roche lobe and have a common envelope (CE). CE’s are gravitationally unstable and transfer mass from the less dense to the more dense star. This can lead to an oscillatory imbalance when mass transfer see-saws back and forth. In O and B star binaries the dance ends when one of the stars goes supernova.

runaway hunting. It was a brief chase. A high-velocity >13 M⦿ was located 4 arcmin away. Sim 147’s nebulosity is ±3° diameter. The SN remnant is an estimated 3000

supersonically through a low-density interstellar medium. Glider and ultralight aircraft pilots experience the same density shock in the subsonic regime when they fly from clear air into and though a cumulus cloud.

Lustrous wallflowers Pulsars revel in the luminance version of anonymity. Simeis 147 is no exception. They emit feebly in any but a few wavebands, mainly radio, Xray, and gamma. Simeis 147’s pulsar, PSR J0538+2817, is as close to anonymous you can get in general astronomy website searches. Aladin Lite (below) reveals emission at its position only in the XMM Newton image set, but in that band the star blazes in the 9.3 arcmin field of ESA / SSC XMMNewton X-ray images at Red=0.5-1 Kev, Green=1-2 Kev, Blue=2-4.5 Kev. It also It emits in the 1.4 GHz radio band at 1.432 mJy. The Landoldt V extinction here is 3.86 magnitudes, which is less an issue here than it is with visual observers, but of great interest to SNR physicists keen to know what exactly they are seeing. PSR J0538+2817 in the XMM Newton 9.3 arcmin field. The star does not present an image in any of the other Aladin Lite bands shown on the partial list to the left.

The WikiSky screen shot below shows the location of Simeis’s supernova remnant, the pulsar PSR J0538+2817. Most pulsars are strong X-ray emitters, but the radiation is not thermal energy from a hot star but rather synchrotron radiation emitted when relativistic electrons spiral around magnetic field lines. The emission direction propagates as a forward-pointing “beaming” cone of radiation around the field line (see ¶4 here). Pulsars have powerful poloidal magnetic fields. Since around 70% of O stars were binaries at birth, in 2015 Dincel, Neuhauser et al searched the nearby field for an O runaway star. The team indeed found an early type runaway star inside SNR S147. HD 37424 is a B0.5V-type star with a peculiar velocity of 74±8 km/s. Tracing its trajectory back via Monte Carlo simulations, they calculated that HD 37424 was at the same position as the central pulsar PSR J0538+2817 some 30±4 kyr ago. This position is only 4 arcmin away from the geometrical centre of the SNR. HD 37424 was the pre-supernova binary companion to the star that went supernova. HD 37424 is 1333 pc (4345 ly) from Earth. The team was then able to calculate that the progenitor to the supernova had a mass of >13 M⦿. HD 37424 is 30 ±4 thousand yr. The team then calculated what the pre-supernova binary was like. They found via

Magnetic attraction is romantic, but not with a magnetar Of the 30 known Galactic magnetars, eight are in supernova remnants. PSR J0538+2817 in Simeis 147 is one of them.

History Roche lobe simulations that the two stars were Simeis 147 was diswithin an AU of each covered in 1952 by other’s surfaces Grigori Abramovich during the late stages Shajn and V.T. Hase at before the progenitor the Crimean Obserwent supernova. vatory on Mount Close O binaries put Koshka, Crimea in on quite a tit-for-tat the former U.S.S.R. mass exchange dance (See more about the via Roche-lobe transobservatory below.) fer. The initially larger The discovery was star donates gas to the made using photograsmaller one until it phic plates taken with swells to larger than a 25-inch Schmidt the first star. The gas camera. flow then reverses Sim 147 was catauntil the newly Magnetar poloidal field lines are so dense that they interact with each other logued by Stewart smaller star becomes at 1015 times the field strength of the Earth’s 25 gauss. Sharpless, an astrothe larger one again. nomer at the Unites The cosmic doe-si-doe goes on until one of them detonates. States Naval Observatory in Flagstaff, Arizona. Sharpless When such an event happens to two very close stars, the compiled his oft-used SNR reference database by examining supernova blasts away around half of the binary’s total Palomar Observatory Sky Survey (POSS) plates for HII mass, sending off its former companion to wander the regions. His second and final catalogue of 313 objects was heavens in splendid solitude. Don’t get in its way. published in 1959. Simeis 147 appears in his list as Sh2-240. The Sim 147 shell is expanding at 80−120 km s−1. Using GAIA parallax data, the pulsar locates the original supernova at 1.47 kpc (4790 ly) from Earth. It extends about 150 light-years at this distance. The remnant is estimated to be about 40,000 years old. The remnant’s fragile, tenuous appearance in the optical and near-IR are consistent with other SNRs.

Simeis 147 was first classified as a possible shell-type supernova remnant (SNR) by Walter Minkowski just one year earlier. At visual mag 9.0 PSR J0538+2817’s runway O companion star is an easy find for a go-to system.

Strolling the Galleria Galactica : Supernovae Remnants

Vela SNR Hα OIII 11-12000 yro.

G34.7-0.4 Aquila SNR bl=Xray, gr-shell, r=MOC.

Tycho Brahe’s 1522 SNR in Cassiopeia.

SNR B0519-69.0 T 1a SNR in LMC 600 yro.

Type 1a white-dwarf SNR B0509-67.5 in the LMC.

RCW 86 Circinis, the oldest known SNR.

Why does Simeis 147 look like this? When we see a painting on the wall, we ask,

1006 case study below). The 40,000 yro Cygnus Loop (Veil Nebula) is in the snowplow phase; its expansion velocity is 120 km s−1. The The reverse shock agitates the vacated interior where the progenitor exploded, which fragmented into many small low-density gas pockets. Since the pockets are thermally unstable, turbulent cells in the inward-moving rebound transfer energy into the irregular density clumps of gas in the old core region. The result is flocculent puffs that we see in both the SN 1006 and Tycho 1572 SNR images below.

“What’s going on in this thing?” (the picture) “What is going around this thing?” (the frame)

In galaxies we ask, “What’s happening to the mass?” “What’s happening to the energy?”

The three phases are generalised examples. Many variants arise because of the diversity of local conditions. The progenitors of Type II core-collapse SNRs are massive hot stars that expel powerful high-velocity winds. A supernova in such a star occurs in a relatively low-density bubble that has been excavated by the pre-supernova star’s intense stellar wind. If the supernova ejecta expands into a low-density bubble it encounters rather little mass ahead of it, hence its free expansion phase is prolonged and the shock shell weakens as it grows larger. When the shell arrives at the more rarefied gas of a spiral arm or interarm region, it weakens into champagne flow, which enters a regime of widely varying atomic and molecular clouds, and local low-density pockets. In most cases, within a spiral arm the interstellar medium is not very uniform, while in the interarm regions the medium is relatively smooth. Local inhomogeneities large and small blur the propagation of both blastwaves and snowplow shells. The parts of a shock front passing through an irregularly clumped volume of gas enter some density pockets conducive to forming a snowplow shell, while other parts of the front nearby may encounter a locally rarefied region, and hence remain in blastwave phase. Moreover, massive stars are born and die while still in stellar associations. The most massive stars go supernova before they have had time to move very far from the cluster or association’s central region. In any large cluster or association the most massive stars will supernova within the first ±20 million years. In an environment in which events spanning a million years are considered “instantaneous”, the expanding shells of multiple supernovae will eventually merge into a common superbubble that can reach sizes in the hundreds of parsecs in diameter. Superbubbles are the primary source of the hot ionized gas in a galaxy’s interstellar medium. See these SILCC sims (1, 2, 3)

Mass is the picture in the frame of Martin’s Sim 147 portrait. Supernova remnants are observed to propagate in three successive phases of expansion: • The first phase is the explosive expansion of the star’s envelope mass as a spherical detonation field. The matter moves outward at high velocity, typically ~10,000 km s-1. It slows inverse exponentially as it spreads out. The remnant associated with SN1987A is in the early explosive expansion phase. The ∼300 year old remnant of Cassiopeia A is nearing the end of the free expansion phase, moving radially outward on average at 6000 km s−1. The internal knots visible in OIII images are oxygen-rich clumps of matter ejected from the depths of the star’s internal convection currents at the time of the detonation. • The second phase of ejection is the blast wave phase. This phase begins when the mass of interstellar gas being compressed in front of the bubble becomes about equal to the mass of the bubble itself. The remnant of Tycho’s Supernova is a Type 1a supernova shock in the blast wave phase (see case study below). The Crab nebula is a Type IIb core-collapse supernova in the blast wave phase; its expansion velocity is ∼ 900 km s−1. • The third phase is the snowplow phase. When the energy on the lee side of an expanding shock front expands into the cloud of gas previously ejected by the star as stellar wind, it forms a dense cool layer directly behind the shock front. This gives a large energy rebound kick propagating back toward the centre of the shell. The 1006 Type 1a supernova recorded as a “guest star” by the Chinese shows how the layers of a snowplow shock front hit the density and temperature shell of the initial shock blast and rebound backwards toward the still-expanding gas below (see SNR 22

Case study: SNR 1006 It's the year 1006 and guess who came for dinner In the year 1006 a "new star" appeared near what is for us Beta Lupii. In just a few days it became brighter than the planet Venus. We now know that the event heralded not the appearance of a new star, but the cataclysmic death of an old one. It was likely a white dwarf star that had been pulling matter off an orbiting companion star. When the white dwarf mass exceeded the Chandrasekhar stability limit, it exploded. Material ejected in the supernova produced tremendous shock waves that heated gas to millions of degrees and accelerated electrons to extremely high energies. Wiki: SN 1006 was a supernova that is likely the brightest observed stellar event in recorded history, reaching an estimated −7.5 visual magnitude, and exceeding roughly sixteen times the brightness of Venus. Appearing between April 30 and May 1, 1006 ACE in the constellation of Lupus, this "guest star" was described by observers across China, Japan, Iraq, Egypt, and Europe, and possibly recorded in North American petroglyphs. Some reports state it was clearly visible in the daytime. Modern astronomers now consider its distance from us to be about 7,200 light-years. The 1006 supernova produced a shell-type remnant. In these, the initial detonation hurls hypersonic star ejecta in a roughly spherical ball. At first the ball expands at up to Mach 20, or >10,000 km s-1. This is the free expansion phase and it lasts many hundreds of years. Eventually the ejecta cools to about 200 K and is expanding 20 km s-1. The shock front continues to expand, but the internal heat is no longer driven by the pressure of fiercely hot gas trying to come to thermal equilibrium with the space around it. For the next 10,000–20,000 years. the gas expansion is driven by adiabatic cooling, expanding at a pace which keeps the interior temperature nearly constant.

Located about 7,000 light years from Earth, SN 1006 was likely a mass-transfer binary in which a white dwarf star that had been pulling matter off a larger but lower-mass companion star exceeded the stability limit known as the Chandrasekhar limit and detonated. The bright outer shock front does not spherically encircle the remnant because of the orientation of the interstellar magnetic field is roughly perpendicular to the visible shock front. Immediately behind the frontal shock wave is a dense counter-shock or reflection wave pushing backwards towards the centre of the remnant. The forward shock experiences temperatures in excess of 10,000 K as it expands into the thin gas of space, but the reverse shock slams into the ejecta still flowing outward. The backshock’s temperature soars into the millions of degrees K. The fluffy red features seen throughout the interior are dense gas pockets heated by the reverse shock.

Eventually the shock front slows to 20 km s-1, which is the average speed of sound (Mach 1) in the interstellar medium (ISM) in a spiral galaxy. When a shock front goes subsonic it slowly diffuses into the general gas medium of the galaxy, the Disappearance phase. Overall, the entire process can take a million years or more.

Case study: Tycho SNR 1572

Source: Chandra X-ray observatory. Recently the Chandra telescoped recorded odd-looking stripes in the Tycho supernova remnant that had not been noted before. Image analysis and computer modeling showed that these were the probable origination source of the most intensely energetic cosmic rays known to science. Above, the blue circle on the edge of the circle represents the outer shell of the blast wave of the supernova remnant. The lighter features are the stripes. The upper square box to the right is a close-up of a region far away from the stripes; the black lines show twisted but not tangled magnetic field lines. The red squiggle shows an electron spiralling around one of these lines.

Source: Chandra X-ray observatory.

Electrons with energies of a trillion electron volts (1012 eV) produced the X-ray emission seen by Chandra. The middle panel shows a close-up of a faint stripe. The magnetic fields are much more tangled and the particle motions are much more turbulent, producing higher energy X-ray emission. In the bright stripe the tangling of the magnetic fields and the turbulence is even stronger. Very energetic tangled fields do not radiate efficiently, but are believed to be the origin of the most energetic cosmic rays in our galaxy.

We’ve come a long way since Tycho Brahe recorded an unusually bright star in 1572. Today we know such “guest stars” are mostly supernovae or bright novae. SN ejection clouds like this and the many others shown earlier are abundant sources of newly-formed elements. These “metals” are why we can breathe air made of oxygen and nitrogen, and use carbon atoms n more different ways than we can count. SNRs are also the primary source of cosmic rays which hurtle around space, packing outsize amount of mass into their tiny protons.

El Nath

White dwarf Type 1a supernova shock fronts like Simeis 147 show us how cluttered “empty” space really is.

Why does Sim 147 look like a see-through cumulus cloud?

Mentally freeze-frame a boiling kettle and you have a pretty good image of the invisible density structure of “empty” space. Instead of steam bubbles, space has gas/dust bubbles. They don’t rise to a top, they swim in a thin soup. We’ve been conditioned to think of interstellar space as a steaming consommé of cold atoms and hot photons. In reality the space between stars — e.g., the vast volume between us and Alpha Centauri — is more like a watery vegetable broth. It is filled with long, slender, noodle-like dust filaments, wraith-like atomic clouds hundreds of times the diameter of the Solar System, egg-yolk sacs of molecular hydrogen clouds surrounded by an albumin of atomic hydrogen. Not to mention the 411 to 413 photons spanning radio to gamma energies zipping through any given cubic centimetre every second. (The numbers differ because so do opinions.) The mechanism of Type 1a SN explosions is far from settled science. See here, here, and there; and videos 1 (adiabatic expansion) and 2 (surface He ignition).

Reverse Shock Waves Moreover, the motions of dust filaments don’t necessarily relate to the motions of gasexplosion bubbles. of Dust filaments and gas bubbles tend to co-exist The supernova a star ejects most (Type II), and in some cases all rather commingle. Dust at particles areof aerodynamic. They flythe (Type Ia),than of the star into space speeds in of space millions miles per hour into through the interstellar medium under the influence of magnetic fields, circumstellar gas. This event generates shock waves that produce shells of while magnetism doesn’t affect gas it is ionised. Some dust hot gas and high-energy particles thatunless are observed for hundreds andis siliceous (sand, basically) because it was seeded with silicon from Type 1a thousands of years as supernova remnants. The supersonic expansion of the supernovae late-stagegas AGB stars. A alarger proportion (57%)that is speeds ejecta into the and circumstellar generates forward shock wave carbonaceous because it formed from carbon molecules in the atmospheres ahead of the ejecta. Like an extreme version of sonic booms produced by the of low surface-temperature AGB giantsshock and Cepheid variables. The two supersonic motion of airplanes, thered forward wave produces sudden, types co-mingle but don’t respond to space’s multitude of forces in the large changes in pressure and temperature behind the shock wave (purple). same dust blobby and amorphous while siliceous dustand is The hot,way. highCarbon pressure gasis(purple) behind the forward shock expands needle-shaped and responds to magnetic fields. Carbon dust is soot that pushes back on the ejecta, causing a reverse shock that heats the ejecta plays a Eventually, vital catalytic in theshock formation thetraverse molecular (orange). therole reverse wave of will the hydrogen cool ejecta which collapse into stars. (Atomic hydrogen can’t make stars). Dust (blue) and heat it. An observer surfing along with the front edge of theisejecta vapourised as protostars are being made. 26

The inverted-colour image above was produced to emphasise the bubblyfoam appearance of Sim 147’s outermost shock fronts. When a Type 1a supernova detonates, its shock wave propagates at high velocity as a hot spherical compression wave. In some cases, e.g., the Bubble Nebula near M52 in Cassiopeia, the sphere expands into a low-density region of space with few atomic or molecular cloud overdensities and little magnetic field pressure. As a Type 1a SN shock expands outward it cools and slows down. When it is dense, hot, and fast, it plows into gas/dust overdensities and compress them into hot lenses with the same curvature as the shock front. Dust filaments respond to an incoming hypersonic shock wave by compressing into remarkable thin sheets (tens of centimetres thick) that are pushed to the very front surface of the shock wave. That is why long-slit spectra of shock fronts acquired perpendicular to the curve of those bulbular cumulus bubbles in the above image show the signatures of alpha and transferric elements instead of H1, Hα, and H2. The leading edge of a gas bubble being blown outward by a shock wave heats to incandescence, which is how we know where the shock wave is. The front surfaces of expanding shock wave the brightest part of the bubble — hence their prettily lace-like look in astro-images. Eventually, though, the shock will cool and slow down enough that the it is deformed by any gas overdensity into which it billows. The overdensity, too, will deform, and will respond by bouncing backward into itself, compressing to densities sometimes great enough to make small star clusters. That is why powerful supernovae spawn a round shell of small star clusters. The process is called triggered star formation. Do this fifty times across a fifty light years and the outermost edge of the original shock sphere looks a lot like Sim 147. That wobbly set of crescents and striations in the picture above was once a perfect round sphere. It takes but an instant for a supernova shock wave to appear. It takes a million years for it to disappear. If you wonder what all this feels like to Luke Skywalker, ask a small airplane pilot to fly you into a cumulus cloud on a hot summer day. Tighten your seat belt. You can be forgiven for imagining yourself alongside Luke as you whoop it up on a helluva hyperwarp through Sim 147.

Dust is just as messy in space is as it is in your house or your optical system

2D simulation of aerodynamic grains in supersonic turbulence. In this simulation by Hopkins & Lee 2015, the grain sizes are 0.001 to 1 Îźm and Mach number is M5. At the instant this â&#x20AC;&#x153;snapshotâ&#x20AC;? in the simulation was frozen, Mach speed and magnetic energy were in equilibrium. The colours show local gas densities relative to the mean gas density of the parcel (blue). The thin black steaks show how the compression and rarefaction of a multitude of small shock fronts (turbulence) tends to flatten a rather chubby filamentary network of dust particles into very thin sheets. Notice the fractal-like coherence in filament patterns. These show that turbulent shocks affect dust pretty much the same on large scales and small. Nearly all the dust lies along razor-thin filaments. In this simulation the dust filaments are not associated with the gas clumps.

Where does a galaxy’s dust come from? Cepheid variables are an often overlooked source.

Dust is one of the least-told stories in popular accounts of why things in the sky look the way they do. Dust is most often described as carbon nanoparticles smaller than soot or cigarette smoke (technically 0.001 to 0.1 µm) that originate in the cool atmospheres of AGB red giants. Latestage AGB stars dredge up large amounts of nuclear material from their cores. Their outer envelopes become abundant with carbon, nitrogen, oxygen, and other low-mass α elements. But that doesn't explain the presence of siliceous dust, which is mainly silicon, calcium, and sulphur. Some of that comes from lateAGB stars, some originates in Type II supernovae, and some comes from the gas expelled by winds expelled from the surfaces of Cepheid variables as they expand and contract. This striking Hubble image of the Cepheid variable RS Puppis is a vivid study in Cepheid mass loss through wind ejecta. Classic Cepheids are 10 times more massive than the sun and 200 times larger. RS Puppis’s surface gravity is only ~0.016 of Earth’s (and about half that of the Sun’s). Its higher-velocity ejecta escapes ballistically into the Galactic medium, forming a huge cocoon of dust. Some particles will escape forever; slightly lower-velocity ejecta will arch high above the star’s disc, slow, and fall back into the maelstrom. The atoms that escape merge into the Galactic medium. Over great periods of time the atoms absorb enough incoming electrons and photons that chemical reactions produce complex molecules. The energy consumed in these processes have a cooling effect in the Galactic medium. By virtue of their absorption they remove the energy from incoming photons and electrons, cooling their surroundings. Star formation requires intensely cold conditions in the depths of molecular clouds. Those clouds are abundant with dust, which does two things: (a) acts as a coolant that slows particle velocities down to gravitational star-forming energies, and (b) eventually deposits their complex mix of elements into new stars, which end up with a higher metallicity than the generation that came before them.

RS Puppis is a beauty, and it tells a great tale. (1, 2, 3, 4.)

Watch how an AGB giant seeds the Galaxy with dust here.

Jaco Brink’s Mission Impossible

N49’s unique filamentary structure has long set it apart from other supernova remnants (SNRs). Most SNR’s appear roughly circular in visible light (see the Simeis 147 article in this issue). The N49 remnant looks to misshapen because it is expanding into a local medium that has wide variations in its gas density. The “empty” space in galaxies is actually rather thickly populated, but most of the gas blobs are cold and do not emit light. Hence N49’s asymmetrical wrung-out-rag look.

The N49 supernova remnant in the LMC

A picture is worth a thousand words. 2,479 of them in this case. The supernova remnant N49 (aka SNR J052559-660453) is only a few thousand years old. Because N49 lies in the Large Magellanic Cloud it took the light in this image over 160,000 years to reach Jaco Brink’s telescope. N49 is a supernova remnant (SNR), the debris left over by a massive star whose overburdened core collapsed in less than one second into a neutron star. The huge mass of the star’s overlying gas collapsed at velocities above 10,000 km sec-1. It squashed into the neutron star, reaching densities almost as immense as the neutron star itself, which is a few times denser than the nucleus of an atom on Earth and in space. The shock rebounded back out at

Many years ago I read a book in which there was a picture of N49. It stated that the neutron star left after the supernova was an exotic kind of neutron star called a ‘magnetar’. Since then I’ve been fascinated by that object. But I never knew where the remnant was located. A couple of weeks before I took this image I started researching N49 again. I discovered that N49 was located in the LMC. I searched online to learn its angular size and magnitude. To my delight I found that it might be within my imaging range. I could hardly wait for the next clear night to take a crack at it. In midDecember 2018 while the Moon was not yet in a crescent phase I used Aladin to find the coordinates of N49 – and also as my reference image when I started imaging it myself. Since I live in a very light polluted area suburb of Johannesburg, I was pleasantly surprised to see the nebula showing up on the view screen of my camera after only a 13-second exposures. I spent the rest of the evening capturing multiple light frames. My imaging scope is a Celestron C11 f/10 SCT on a Celestron AVX mount. The camera is a pretty straightforward Fujifilm X-T1 with no mods for astro work. I do not yet have autoguiding, so the maximum exposures I can take are about 13 seconds before the star images became elongated. Altogether I captured 958 images of 13” each at ISO 3200. I set the camera’s intervalometer to capture 20 frames at a time, Then I re-centred the FOV and then repeated the sequence. Next day I took 50 flat frames and a matching number of 50 frames. That evening I took 958 dark frames. I use DSS 3.3.2 for my stacking. Since DSS does not read my camera’s RAW files properly, I had to import all the images to Lightroom, then export them as TIFF files, which I then imported into DSS. I expected hiccoughs stacking 947 light frames to produce the final image, but was delighted (and relieved) when I found myself looking at the image you see here. A bit of Photoshop’s levels colour and saturation adjustments to pip up the final result, and here we are.

7200 to 9600 km s-1. All this happened so fast that the surface of the star had no idea anything was amiss until the shock wave arrived at its surface several minutes later. This type of shock is called a detonation because the wave is speeding at many times the local sound speed. If a shock front is propagating subsonically it is called deflagration. At this point it can accurately be said that all hell broke loose. The exploding shock wave first sped into the rather thick and sludgy shell of dense gas that had collected around the star for tens of thousands of years. Over the next few thousand years the shock wave slowed to 100 – 300 km s-1. All these pressures and densities created massive hot spots and dense filaments, which brings us up to today when Jaco set up his camera and merrily started shooting away. A decade ago images of N49 by the Chandra X-ray Observatory revealed a bullet-shaped object traveling at about 8.1 million km hr-1 (2250 km s-1) away from a bright X-ray and gamma-ray point source. The had the earmarks of a neutron star with an extremely powerful magnetic field, or magnetar. Objects with this behaviour and orogeny are often soft gamma repeaters because their magnetic fields are twisted by the star’s rotation until they periodically release the pent-up energy in a blazing flash of photons so bright we see them mainly in gamma and hard X-rays.

=Jaco Brink

The gas shocks we see are is about 75 light-years from one side to the other. N49 is the brightest supernova remnant in the Large Magellanic Cloud. It still has million-degree gas in its centre but cooler gas at the outer parts. “Cooler” is a relative term: here it can be somewhere between 8,000 and 300,000 K.

On March 5, 1979, the R49 magnetar emitted a titanic and still incompletely understood gamma-ray burst. The energy packed into a photon is inversely proportional to its wavelength. Gamma photons are shorter than the width of an atomic nucleus. They have a million or more times the energy of visible light photons, which is why they are so good at worming their way between protons and neutrons in fragile nuclei like Uranium 238 to split them apart and make atomic bomb blasts. The neutron star in N 49 has had several subsequent gamma-ray emissions. It is called a soft gamma-ray repeater (SGR) for this reason. SGRs are an oddball subclass of neutron stars. They produce gamma rays that are less energetic than most gamma-ray bursters, and they repeat the show on irregular time scales. The SGR’s neutron star spins one revolution every eight seconds. Its

The yellow-white filaments in this image were acquired in July 2000 using the Hubble’s Wide Field Planetary Camera 2. Colour filters were used to sample light emitted by sulphur, oxygen and hydrogen. Source: Hubble Heritage Team (STScI/AURA).

magnetic field is about 1015 (a million billion) times more powerful than the Earth’s. Your fancy smartwatch would be ruined if you were approaching from the orbit of Uranus. M49’s magnetar zooms through the supernova debris cloud at over 1,200 km per second. The star also emits X-rays whose energies are slightly less energetic than soft gamma rays. Highresolution X-ray satellites have resolved a point source near the centre of N 49 as the likely X-ray component of the spectral signature of the soft gamma-ray repeater. This helps locate the original supernova site because it cannot be traced using gamma rays due to the design limitations of grazing-incidence gamma ray telescopes like Chandra. (See the X-ray version of the design here.) The delicate filaments and knots throughout the supernova remnant — visible in IR, the visual band, and X-rays — are sheets of debris from the stellar explosion. This filamentary material will eventually be recycled into new generations of stars. Our own Sun and planets are constructed from similar debris of supernovae that exploded in the Milky Way billions of years ago.

This composite N49 image combines Hubble Space Telescope optical light with Chandra X-ray imaging in blue and Spitzer data in red. The Chandra satellite camera was designed to capture high-energy short wavelength photons generated by gas in the central regions in the region around the original supernova (now a neutron star). This gas was heated to millions of K by the SN shock wave and has lost little of that heat â&#x20AC;&#x201D; more properly, particle excitation â&#x20AC;&#x201D; since. Its present temperature is in the million-degree K range. How does gas in regions like this stay so hot? In the supersonic shock and twisted magnetic regime of gas heated by a SN shock wave, gas has very few ways to bleed off the energy imparted by the blast wave. The main method of gas cooling in such a regime is particle collision in which two particles, e.g., a proton-neutron or electron-neutron pair, convert some of their pre-bounce energy into slower rebound velocities. The atoms are simply too far apart to collide very often.

When they do collide, both particles lose some velocity, and emit a photon. Those photons are what we see as the pinkish glow. The outer parts of the remnant, being more distant from the blast and the passing wave, had cooled and slowed by the time the shock wave reached them. Today the outer regions have cooled to infraredemitting temperatures. The Spitzer space telescope was the preferred instrument for capturing IR photons at the time this image was assembled. Those photons were interpreted as blue in the image. The Hubble telescope captured the SN remnantâ&#x20AC;&#x2122;s filament-like shock patterns (white & yellow here). The tremulous interlaced quality of SN remnant filaments reflects several ambient conditions. One is pre-existing pockets of dust squeezed from blobs into wavelike ropes by the blast wave. Another is a complex interaction of magnetic fields and free-flowing electrons originally set free by the passing shock blast. A third

explanation for the twisty mess is the way small local shock waves interact with other when there are a great many supersonic shocks mixing it up in a given volume. All by itself a supersonic wave expands spherically. But when dozens of them exist at varying strengths, velocities, and stages in their expansion, each wave front collides with others in very intricate ways. The net effect over time is to distribute pressure more evenly and reduce overall tumult. Next time you boil a pot of water, before you put the veggies in to cook, look closely at what happens to the shapes of the bubbles and they pop out at the surface of the water. Watch how they merge, interfere with each other, and pop. Now go look at one of those fabulous HST shots of fine details in the Crab Nebula M1. Whether on the surface of a boiling pan or in a given volume of a shocked gas cloud, multiple shock collisions act to relieve internal pressure differences. Its hot at that surface, but also smoother than the seethe of upwelling bubbles below. In M49 much of the energy propagates in the optical bands. In the image above the optical emission is shown in yellowwhite. All this raises a question. Why did the astronomers who planned on this image choose these particular telescopes and the bandwidths of the filters they needed to pictorialise the energy levels so they come out like this image? Little ink has been expended in the amateur astronomy journals about how the ravishingly beautiful pictures on their pages were planned in such a way that they would come out looking the way they do. When groups of astronomers prepare their proposals for telescope and computer time to acquire their various data, they are really looking for the best way to capture the energy levels that will answer their questions about physical activity in the object. But first they have to plan ahead by looking behind: How well did images in the past record capture the desired information, and how could they design the next image to do a better job? For example, in the Chandra/Spitzer/HST N49 image above, if one of the team members wanted data on synchrotron radiation from X-rays generated by electrons spiralling at near-light-speed velocities around magnetic field lines, they want telescopes that capture photons with energy levels of 0.1 to 1.0 keV (thousand electron volts). It so happens that XMM Newton (still operating,

BTW!) and Chandra record those energies. Voila! That’ll be 3 hours, please. Then the astronomers have to decide which energy level they want that both delivers the info but also does not overwhelm the data which their colleagues want in the IR and visual. There’s a lot of give-and-take in these colloquies. Plus, they usually aren’t able all this telescope time on one night. It might take months before any particular instrument has enough time to gather the desired info. That is one reason why so many astronomers have multiple projects underway at any given time — and why their names appear as co-authors in so many papers published in the astro lit in any given month. If you can’t wait another minute before nailing your own Chandra or Hubble Telescope proposal for an observing slot, visit Liénard-Wiechert Field. Take the dog out for a walk first. Early in N49’s days in the crosshairs of interested astronomers it was tagged as a breed apart from other well understood supernova remnants. Most supernova remnants appear roughly circular in visible light. Early mapping of the molecular clouds in the region surrounding N49 suggested that the supernova remnant was expanding into regions of varying density, particularly to the southeast. Differential expansion would explain its asymmetrical appearance, but also its non-uniform temperature regime. It’s not a sphere, OK, but the remnant is cooling in thermal isopleths that didn't match the filamentary structures that we see in the optical. Hence something is causing what amount to heat shimmer (IR) that doesn’t match its optical shimmer. X-ray data show the SNR brightening in the southeast, confirming the the remnant is colliding with denser non-emitting clouds in that sector, but the IR thermal shimmer is not fully understood. What we DO understand is that Jaco has given us a good enough image to cut some research teeth on. Thanks, Jaco! _________ If you can’t wait another minute without learning everything there is to know about photometric choices to provide specific data, here are a couple of good places to start: Univ. of Rochester Lecture on filters and photometry; ; and of course, we can’t leave out everyone’s favourite XYZ-Made-Easy resource, Wiki (go down to “Synchrotron radiation in astronomy”).

Why are comets so green?

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α β γ δ ε ζ η θ λ 2 ι κ ν ξ 7 σ τ φ ψ χ ω ϕ Ѳ‖ ‡ † ∏ ∂ ∆ ∑ ∞ ⨁

Martin Heigan

As the comet gets closer to the Sun, somewhere around the orbital diameter of Mars its surface heats up to the point where the comet’s tiny dust particles are sputtered loose with enough energy to escape into space. Such particles make a second “dust tail”, usually yellow/white in colour. The dust tail reflects enough light that spectrograms can reveal the precise chemical makeup of those particles. Comets are assembled from the complex molecules that existed in the solar system at the time many other Solar System bodies were made, roughly 4.57 billion yeas ago. That makes them superb chemical probes of the galaxy’s hydrogen, oxygen, carbon, and nitrogen mix in that era. Two molecules of particular interest to cometwatchers are the cyanide/cyanogen (CN) carbon-

The image of Comet Wirtanen on the previous page was taken by ASSA astrophotographer Martin Heigan.on XX. His processing reveals a greenish hue that is not as obvious to the visual observer because our eyes do not accurately render colours when light levels are low. Some people can see the Orion Nebula as a faint greenish hue, yet others don’t see that at all. Because of the long total exposure times of astro-images, colours become more apparent, just as faint nebulosities and stars do. Comets are a mix of ices, rocky conglomerates ranging from grain-sized nubbins to extraterrestrial Gibraltars, all coated with tiny space dust particles smaller that the particles in cigarette smoke. “Ices” are not just the H2O water ice that we see in ice cube trays, but also components like dry ice (frozen CO2), methane

nitrogen group, and diatomic carbon (C2). The word (CH4), ammonia (NH3), carbon monoxide (CO), “organic” in astrochemistry means that the compound plus a veritable chemical warehouse of silica-and has carbon in it. Free 12CO and 13CO are key tracers of sulphur-based compounds. Not to mention atomic hydrogen clouds that emit only weakly in the polycyclic aromatic hydrocarbons (PAHs) whose radio 21 cm radio band but are awash in 12CO and composition interests planetary astronomers 13CO that glow brightly in the microwave band; the because they reflect the chemical composition of space at the time the comets, planets, and stony ratios between them are used as tracers of **Sigh** – It was inevitable. debris of our solar system were formed. temperature and velocity. Out beyond the orbital circle of Jupiter, space A comet’s teal or blue-green colour comes from is so cold and photons so few that the ices remain frozen. As comets enter gases stimulated by ultraviolet light when bound electrons are boosted to the increasingly rich photon and gas broth nearer to the Sun, they heat up. higher energy levels, but then drop back to lower energy levels. Some of those The number of energy-laden ultraviolet photons striking the comet transitions result in an emission line that falls in the blue-green part of the surpasses the threshold where electrons can be knocked free from the outer electromagnetic spectrum to which human eyes are sensitive. When we see electron shells of certain atoms, ionising the atoms. The compound most that green colour, it indicates that the coma contains cyanogen (CN2, or more vulnerable to ionisation is the weakly bound carbon monoxide or CO exactly N≡C—C≡N) and C2 molecules. These show up when the comet is molecule. Incoming UV photons pack enough kinetic punch to make CO+ outgassing as it nears the Sun. Despite the prefix “cyan-“, cyanogen isn’t molecules. They stream directly away from the comet in what we observe as lethal like its chemical relative cyanide (CN)2. The “cyan” derives from the a blue ion tail. Faint blue tails are the first signature of a comet warming up. same latinate root as the optical colour.

Known but unknown for 48 years Comet Wirtanen acquired its name from the astronomer Carl Wirtanen in 1948. He specialised in Solar System physics. He earned a reputation as a skilled comet/asteroid spotter after devising his own system of scanning weekly photos of the night sky using a blink comparator to quickly spot moving objects. Blink comparison was a well known technique (Clyde Tombaugh discovered Pluto that way), but Wirtanen’s approach was persistent and methodical. This was how he spotted and soon calculated the path of an unknown comet-like object in 1948. The orbital path he plotted over two weeks suggested that he was looking at a periodic comet with an orbit of 5 to 6 years. Its returns were recorded but unstudied between then and 1996. In that year a team led by Karen Meech of the University of Hawaii used a 2.2 metre telescope to image and take spectra of the comet during its relatively nearby Earth crossing that year. Its apogee was calculated to lie slightly exterior to Jupiter’s orbit and its perigee lay approximately the Earth’s orbital diameter.

Comet 46P/Wirtanen’s orbit keeps it fairly close to the sun. Its aphelion, or farthest point from the sun, is about 5.1 astronomical units (AU), which is slightly larger than Jupiter’s orbit. Its perihelion, or closest approach to the sun, is about 1 AU, which is the Earth’s distance from the sun. A full orbit takes 5.4 years. This image incorrectly suggests that the comet’s orbit does not reach Jupiter’s 36

How do grand plans turn into telescope time? The Wirtanen Observing Campaign. From the moment 46P/Wirtanen’s third return visit after its 1997 appearance was first spotted in images recorded by all-sky survey telescopes on 18 June 2018, planetary scientists saw the return as a historic opportunity to marshall the best available instruments and analysts in an all-embracing study that invited amateur astronomers to contribute. The Galaxy Zoo “citizen science” idea was bearing fruit in distant orchards. Wirtanen’s closest approach to Earth at 0.077 AU would take place on 16 December 2018, just four days after the comet’s perihelion (closest approach) to the Sun. That made it one of the nearest comets in modern times. Its near-Earth trajectory meant it would be very bright, Early guesstimates suggested it could reach naked eye visibility. The Wirtanen Observing Campaign wirtanen.astro.umd.edu knit together a team of over 100 professional and advanced amateur imagers and spectroscopists using 46 different instruments around the world. Their main goal was to refine and improve the evidence of the two-decades old study led by Karen Meech of the University of Hawaii. Her team had observed comet 46P/Wirtanen primarily during its more distant Earth crossing in 1996. Astronomy facilities and calculational prowess had evolved enormously during the 22-year time lapse.

Wirtanen’s projected orbital trajectory with respect to the Earth meant that it would be in observing range for most of a year.

Although Wirtanen was discovered in 1948 its only approach close enough to Earth for accurate spectroscopy occurred in 2013, when it was 907 million km from Earth. Fine-grained details of the coma nearest the rocky surface eluded astronomers, yet this information was the most important, Comet ejecta are important seedbeds of interplanetary gas composition across billion-year time spans. Whatever gases come off Wirtanen today closely correlate with overall species abundance ratios in the Earth–Jupiter belt going back millions of years. Comets are frangible. To paraphrase Yeats, things fall apart, the centre cannot hold. Jupiter is often involved in orbital shifts while the Sun can break them to pieces. Comets add to their own woes with episodes of violent outgassing, as we saw in Comet Holmes in 200X. If the gas expulsion is one-sided it can alter the comet’s orbit. The closer a comet’s complete orbital ellipse is to Jupiter and the Sun, the lower its life expectancy. Periodic comets tend to fall into two groups: Halley-type with orbits 20 to 200 years, and Jupiter family with orbits less than 20 years and aphelions about the same distance as Jupiter’s orbit. A less-heralded third family is the Centaurs, minor planets in comet-like orbits that never come close enough to the Sun to outgas.

Spectroscopic seismograms The peak lines in spectrograms can be thought of as a sort of seismogram of light. Instead of terrifying tremors of the land we have sighing tremors of the sky. Using the Arecibo Observatory 10–18 Dec 2018, a team led by Ellen Howell detected an extended, asymmetric skirt of large (>2 cm) grains in 46P/ Wirtanen’s coma consistent with a fiducial nucleus size 1.400 x 1.1 km. The image below shows the detection of the nucleus (spike) and the extended skirt of large grains surrounding it. On Dec 16, Dave Schleicher put meaningful numbers behind the various emission peaks in the spectrogram above. On that date Comet Wirtanan was outgassing

Once Comet Wirtanen was re-spotted moving in toward the Sun on June 18, its orbit was refined to predict a Dec 16 closest approach to Earth. Astronomers were elated that it would pass only 11.7 million km away from Earth, only 29.25 times further than the Moon. In the mid-1990s astronomy data crunching and storage capacity on the Internet was in its infancy by today’s standards. By early 2018 the Internet was regarded as the primary vehicle for astronomical analysis in the same way the telescope mirror had come to be seen as the primary vehicle for data capture. The University of Maryland team 46P/Wirtanen observing campaign was set into motion when UMD’s Lori Feaga et al. recovered the comet 18–20 June and circulated its images to cometary specialists around the world. The community responded by formulating the most comprehensive comet-study campaign in the history of the discipline. The

(OH) = 5.37 x 1027 mol/s or roughly 8,917 hydroxyl molecules per second (H2O)= 7.24 x 1027 mol/s ≈ 12,055 water molecules per second (CN) = 1.29 x 1025 mol/s ≈ 214 cyanide family molecules per second (NH) = 3.36 x 1025 mol/s ≈ 557 nitrogen monohydride m.p.s.

campaign was finalised at the 50th Division for Planetary Sciences Meeting in late October 2018. Here is the Powerpoint presentation assembled for that meeting.

Doesn’t seem like much – until you multiply by 86,400 seconds in a day, times the approx. 85 days the comet was outgassing, and that these gases were expanding into a region of space that contains about 10 to 100 atoms cm-1. 37

Does 46P/Wirtanen rotate?

On 21 December 2018 Ellen Howell et al. reported the detection of the nucleus of comet Wirtanen using the Arecibo Observatory. The group derives a nucleus of 1. 4 x 1.1 km and an extended, asymmetric skirt of large >2 cm grains in the coma. The image above shows the detection of the nucleus (spike) and the extended skirt of large grains surrounding it.

On 26 November Dave Farnham et al. reported a rotation period of 8.91 hr, based on the coma morphology seen in this figure. Repetition over at least eight rotations is consistent with a conjecture that the nucleus is in a state of simple rotation. (Click on this link to see an 8frame movie of the comet rotating.)

Two weeks later on 9-10 Dec 2018 Emannuel Jehin et al. reported that a CN molecular light curve measured from photometry obtained at the TRAPPIST telescopes showed a 9.2 hr periodicity, which is assumed to reflect the rotation period of the nucleus.

Another shining hour for citizen scientists Wirtanen’s observing conditions offered unique opportunities. During the comet’s closest approach to Earth, 1 arcsec was less than 100 km on the face of the comet. Even small telescopes of 200 mm and up could access the inner coma at high power. At greater distances, the inner coma of a comet is amenable only to spacecraft or large telescopes with adaptive optics. Moreover, the comet was also diffuse and fast-moving, a challenge for professional and amateur imagers/spectrscopists alike. The WOC project was especially noteworthy for the number of contributions made by citizen scientists — the comet was continuously observed when seeing conditions were suitable from early November 2018 till the end of January 2019. The amateur community provided monitoring and context imaging that yielded valuable rotational light curves and outburst photometry and spectra. The amateurs served on-call at any hour if professionals need quick observing response when their own telescopes were in the middle of monitoring or imaging. 38

Even students went wild about Wirtanen In the U.K., Helen Usher, a PhD student at the Open University and Cardiff University, works with the Faulkes Telescope Project in the UK. This is an effort to coordinate students from schools across Europe to collect and analyse data acquired from Comet 46P/Wirtanen. Ms Usher was inspired by the Rosetta mission to do her PhD about involving school children in astronomical research. She recruited eight schools from the UK, Germany, France and Norway to pool their students’ talents — including one primary school. The Faulkes Telescope Project uses a worldwide network of 2-metre robotic telescopes in Hawaii and Australia that were built and operated by Las Cumbres Observatory. The Faulkes Telescope North in Hawaii is equipped with a series of filters especially installed by the European Space Agency (ESA) to study Comet 67P from the ground during the Rosetta Comet’s visit, and are now being used to observe 46P. The filters were designed to separate the gas and dust content of the coma. .

This unusual image of Comet 46P/Wirtanen looks like it belongs in an art gallery. No, it came from Jost Jahn of the Observatoire de Haute Provence in France. Acquired on 10 December 2018, the image puts the comet in the lower right of the image, with its tail extending to the upper left. The image is a composite of 3x33 short, 15 second exposures using RBG filters on the ROTAT telescope, a coma-corrected 60cm f/3.2 Newtonian using an SBIG STL11000 full-frame CCD. It was taken within the framework of a research project by 11th grade students from Mannheim, Germany at the Hector Seminar, an organisation that fosters gifted high-school students in STEM subjects. The image was processed with a Larson-Sekanina rotational gradient filter to enhance the contrast and reveal the comet’s tail. The diagonal stripes across the image are star trails at two of the rotational settings. ROTAT (Remote Observatory Theoretical Astrophysics Tuebingen) is operated by the German Stiftung Interaktive Astronomie und Astrophysik (Foundation for Interactive Astronomy and Astrophysics) whose goal is to inspire young adults with hands-on science. From the looks of it, those students have done pretty well.

High school students with images of Comet 46P/Wirtanen at Dodderhill Girls School near Droitwich Spa, in the UK. The students are analysing data about the comet as part of Helen Usher’s Faulkes Telescope Project. The Open University in the U.K. and Cardiff University in Wales acquire their data using the robotically controlled Faulkes Telescope in Hawaii. 39

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My interest was drawn to this filamentary dark cloud because of its catchy name. I first came across it in Auke Slotegraaf’s ASSA 100 list. Being only a few degrees SW of the Coal Sack makes it an easy star hop (or rather cloud jump). The Dark Doodad is more referred to simply as “DD”. It got its name from the American amateur astronomer Dennis di Cicco in 1986 after he spotted it on an image he had taken in central Australia. It has several names in the professional astronomy community, but nothing can better “Dark Doodad” as an attention getter. It is best photographed in Karoo-quality skies in a location far from any urban light domes. A few eagleeyed visual observers have seen it, but like faint galaxies, it “pops” into view for only a second or two before melting away. The nearby globular cluster NGC 4372 enhances its aesthetic appeal. Canon 5D3 DSLR, tele Vue 127mm refractor, and a large tack of multiple subs.

I captured these images at the Karoo 2018 Star party. The wide-field shot was taken with a Canon 200mm lens at f/2 on a Canon 1Dx DSLR, unguided but on an excellently tracking a Celestron CGX mount. The second image above shows me getting a precise level on the tripod baseplate, knowing the difficult shots I would be doing that night. The camera was controlled via a Canon intervalometer. The inset close up used an Orion optics UK (OOUK) 10” Optimised Dall Kirkham scope at f/6.8, mounted on a Celestron CGX-L. Auto-guided with a Tele Vue 85mm refractor, Orion Starshoot cam. Processed in PHD.

I often image an interesting object using different OTAs of different focal lengths. One image set puts the object in the context of its Galactic surroundings — near the centreline of the Galactic disc 1.4° W of Gamma Aquilae. The inset image was acquired with a 10-inch 1700 mm f.l. DallKirkham to bring out the delicate structure. The U shape occurs because it was once a more or linear filamentary cloud that was compressed by turbulence in the chaotic spiral arm disc plane until it twisted into the shape we see today. Located about 2000 light years away toward the Scutum Star Cloud region, visually B143 is about the size of the full moon. It is very difficult to spot without optical aid. It shows up well in 15 x 70 calibre binoculars.

I’ve long been fascinated by what I call the “foot” of Simeis 188 – NGC 6559 and IC 4865. Although located only 1° from the M8 Lagoon Nebula, Sim 188 is much nearer to us and is a million years younger. When I read an APOD post about I was attracted by how beautiful was its chaotic mess of emission-, reflection-, and dark absorption nebulae, all mixed in with a sprinkle of open clusters. The bright red NGC 6559 is furiously forming stars deep inside and hidden by the red HII gas. Sim 188 is a smorgasbord of everything we like to explore and record.

While working on the Magellanic Clouds project with Magda Streicher and Auke Slotegraaf, I came to appreciate how many deep sky targets the SMC has on offer â&#x20AC;&#x201C; a real SmĂśrgasbord! This is a bucket list of potential targets I still must visit and image in future.

It never hurts to have a shopping list. This is what I plan for the SMC. When I get around to it, that is. There is another nice Magellanic up there that could use some attention, too.

Why does an imager need so many OTAs? Building a personal library of object types implies wide-field gear for the “Big Picture” shots that show an object in its larger Galactic context; and highresolution capability to get into the detailed hearts of objects. For the Southern Cross and Coal Sack, for example, I use the 200 mm f/2 Canon 1DX DSLR, while planetary nebulae, globulars, and galaxies are best imaged with the 10-inch long efl DallKirkham. Its 1700 mm mandates a great many image, dark, bias, and flat expo-sures. Those in turn require dead-accurate tracking and guiding. The 127 mm TeleVue homesteads the territory in between. The Dark Doodad image was captured using it. Yes, it is a large investment and each setup soaks up a huge amount of time for accurate setup, imaging, and process-ing after. The reward comes only at the very end when the final image pops up on my computer screen and I yelp, “Got it!” It’s a heck of an investment for two short words, but those words are priceless. 51

ATMs in South Africa Dirk Rossouwâ&#x20AC;&#x2122;s Melkbosch Skies Observatory

Dirk Rossouw built his backyard observatory in the Melkbosstrand suburb 25 km from Cape Town

To earn your badge as a DIY stargazer, you start with a hole in the ground.

I was a “late bloomer” with respect to the purchasing of my first telescope. My first telescope was a Celestron NexStar 127SLT Mak. I soon, however realized that it wasn’t enough. I subsequently bought an old Meade XL200 10” Classic and although I was very happy with its performance I jumped to it when I recently had the opportunity to acquire a 14” Meade LX200R GPS. While the 10” scope can still be described “mobile”, the latter definitely is not. This lead to the construction of the Melkbosch Skies Observatory.

Initial planning I am not a handyman. My dear wife wryly describes me as, “Ten thumbs with a husband attached!” Constructing a small backyard observatory to accommodate my needs wasn’t easy. There are no private companies in South Africa, or in the rest of Africa for that matter, that construct backyard observatories. I considered ordering a pre-fabricated POD kit from Sky Sheds.com in the USA. Although they don’t normally ship to South Africa, the chaps were extremely helpful and were quite willing to make a plan. Eventually I decided that the POD would be too small for my equipment storage needs. So I did the only sensible thing: I commissioned a private construction company. Detailed planning for the Observatory continued for a few weeks before construction began. Being in a built-up area with relatively small 500 m² plots, finding a suitable spot for the observatory was difficult. The floor level had to be elevated to see over the adjacent roofs and trees, and there were other limiting factors.

We dug seven holes. Four for the observatory structure itself, one for the pier and two for the columns that will eventually carry the stays for the roof to roll open. All in all we used some 22 bags of cement, 42 bags of gravel and 71 bags of sand. This structure will most probably be able to withstand anything!

Type of Building The most important design choice to be made was roll-off roof vs. dome observatory. Domes have traditionally been the classic design for observatories, but roll-off roof designs provide an all-sky open view. Construction of a flat sliding roof is also less complicated. After review of the offerings for prefabricated huts, mostly wood and plastic, I chose the latest light steel frame construction method with flat roof running on wheels. It is 9 square meters (approx. 10 x 10 feet) in extent, which provides enough space for me, my equipment, and a couple of guests. Once poured, the concrete was left to set.

The first three rules of successful observatory construction are solid, solid, and solid. Base for floor constructed and shutter wood sheets being installed. Floor raised 80 cm from ground level to improve views over adjacent buildings.

Steel columns supporting the channels on which the roof will eventually roll open.

Construction We opted for a light steel frame construction on heavy steel base. The very able construction team of Global Construction Management were responsible for the actual construction of the Observatory. Des Palm, the owner of the company. predicted at the outset of the project, we would have a number of interesting “Oh sh@#!” moments throughout the project. We never, however failed to overcome any of these. The floor was constructed on 306 x 306 mm UC steel columns, raised 800 mm from the ground. The floor surface consists of 21 mm shutter board flooring, with 25mm Isoboard insulation and 12 mm MagnaBoard sheets on top. I installed industrial grade carpet tiles on top. The walls are 12 mm MagnaBoard (Magnesium Oxide) panels onto 90 mm x Z275 galvanized Light Steel frames with 63 kg mineral wool wall insulation and 18 mm shutter board on the inside. The corner posts are 150 x 150 x 6 mm steel columns. The roof is Colourbond steel roof sheeting on industrial sisalation on 100 x 50 mm galvanized lipped channel purlins on 100 x 50 mm rectangular roof beams.

The exterior MagnaBoard panels were assembled with ample space between the inner and outer walls to install coax conduits for services. Thick layers of aluminised wall insulation filled the gap between the walls.

The exterior panels were painted with base coat followed by high quality Weather Guard outer paint to blend with the residence’s outside walls. The interior walls are wooden shutter board panels. I treated the wood with three coats of clear Woodoc Indoor Polywax Sealer with a satin gloss. The electricity conduits within the wall and the port where the computer and video cables exit from under the floor from the telescope pier.

Utility Services It made sense to install utilities at the outset than to add them later. For this reason, I planned on ample electrical and data/video services with provision for expansion. The major elements are: • Eight circuit breaker distribution board with space for expansion if required • Eight wall plugs installed in all four walls, with three in close proximity of my work station • Special wall plug for roof motor • Switched electrical outlets on each circuit, to control interior LED lighting • Video cabling, to support video cameras and for LED video monitor (actually small TV) • Conduits to the ports in the pier and wall plugs to eliminate trip hazards.

The observatory door had to be oriented to the north-east to keep the door opposite the prevailing south-wastern wind and also facing the residence. We have since decided to enclose our patio area, with the result that the observatory door now forms part of the enclosed patio area, thus improving access tremendously. The roof opens and slides away to the south. We initially used a standard 500kg gate motor but soon realized that it wouldn’t last with the heavy roof. We eventually opted for a workshop hoist crane motor that the contractor had available. Although rather too strong for this usage, I will never have to worry about not being able to open or close the roof.

1 000 kg (1,6 KW) electrical hoist motor adapted to open and close the roof. A gear was specially engineered to fit the motor and tracks for the roof. We had to install steel braces behind that part of the wall to handle the motor’s movement.

Pier head and adjustment plates laser cut by a local engineering shop. All steel, including steel columns galvanised and subsequently painted with rust resisting Hammerite steel paint.

Telescope Pier The telescope pier was constructed of heavy steel, 150x150x6 mm emplanted in a 1 cu. meter steelrenforced concrete. An opening in the floor slightly larger than the pier prevents floor vibrations from telegraphing to the pier. The hole is also a convenient aperture for the electricity, video, and computer cables. To keep electrical cables from littering the floor, they were channeled under the floor in separate conduits to rise inside the pier. They attach to a multi-plug distribution box on the side of the pier. I bolted a heavy-duty Meade equatorial wedge on top of the pier for long exposure imaging sessions come the day my interested evolve that direction. Since I already had the telescope, it was easy to measure and calculate the optimal pier height. I held my breath the first time I closed the roof with the telescope in the â&#x20AC;&#x153;parkâ&#x20AC;? position, but was delighted to see it clear the top of the telescope by three cm when the telescope is pointed at the horizon. Although I initially thought that the base was sturdy enough to prevent vibrations, I discovered that vibrations took about 3 seconds to dampen when the scope was bumped by accident. The solution was to not to weld larger and longer flanges onto the base but rather to encase it in a concrete pillar.

Note wall plugs in the background, plugs and video/ computer box on the pier shaft and heavy duty Meade equatorial wedge installed. The blue heavy duty carpet tiles provide a nice finish.

Red LED strip lights with roof closed. The aluminum strips were painted the same colour Hammerite paint as the steel finishes, so as not to reflect light in my eyes when the lights are on.

It didn’t take too long for the distaff side of the Rossouw residence to notice that my new observatory was almost the same height as the living room floor of the house. It took even less time for me to be convinced what a topping idea it would be to extend the living room into a sun room attached to the stairs to the door of the observatory. Thus it came to be that in our house the “good life” is defined as my wife having her own sunny recreation and me having my starry hideaway. Note the Observatory’s double doors open into the room. No more braving of the winter elements to get to my observatory.

A job extremely well done! And a glass of South Africa’s finest Pinotage’s to celebrate the occasion. Without Martin’s invaluable assistance I would have battled to get it aligned so quickly and effectively.

First Light The Observatory structure was completed by end October 2017. The telescope was installed late December 2017. On the night of Saturday, 13 January 2018 the telescope and observatory officially saw its “First Light”. My good friend and astronomy hardware mentor, Martin Lyons and I duly celebrated the occasion with a glass of very good Pinotage.

http://melkbosch-skies.com/ portfolio/observatory/ http://melkbosch-skies.com/ 59

dirk.rossouw@gmail.com

If ever there was a visual exemplar of Occam’s Razor, this is it. The Dutch cosmographer Andreas Cellarius is best known for his spectacular celestial atlas, the Harmonia Macrocosmica. This plate and the next are from that work. Cellarius’s atlas described ancient and contemporary astronomy as interpreted by the theories of Ptolemy, Copernicus, and Tycho Brahe. Cellarius’s rendering of Brahe’s modified version of Ptolemy’s epicyclic interpretation of celestial motions was an elegant summation of the unintended consequences of axiomatic logic. The axiomatic theory postulates that one is obliged to derive every aspect of a thought system from a single fundamental assumption and a single law of propagation – e.g., western linear logic. In Brahe's geo-heliocentric system the sun and the moon orbited the earth while the remaining planets revolved around the sun. This chart depicts Brahe's model encircled by the twelve signs of the zodiac. The map is grandly titled Planisphaerium Braheum, sive Structura Mundi Totius, ex Hypothesi Tychonis Brahei in Plano Delineata. Cellarius even threw in Brahe himself as the munificently moustached gentleman in the lower right corner. Source: Planisphaerium Braheum 3/28/2018.

A Dialogue between

two world systems

This striking example of the Copernican model of the Solar System was printed in a 1708 Valk & Schenk edition of Andreas Cellarius's Harmonia Macrocosmica. Andreas Cellarius’s version of Copernicus’s heliocentric theory is regarded today as one of the most spectacular cosmographical atlases published in the 17th century. It was first issued in 1660/1661 by the Amsterdam publisher Johannes Janssonius (1588-1664), as a cosmographical supplement to his Atlas Novus. Andreas Cellarius had already started the atlas in 1647 and intended it to be a historical introduction for a two-volume treatise on Copernican cosmography but the second part was never published. Cellarius' rendering of the Copernican concentric circles summarised the apparent movements of the planets around the sun, the moon around the Earth, and Jupiter’s four known moons around the planet itself. The original copper plate engravings were uncoloured. An entire industry of hand-colouring printed copies of engravings grew up to satisfy demand for more vibrant illustrations. Source: Barry Lawrence Ruderman Antique Maps.

The Beauty in Our History Humans have always been fascinated with the solemn parade of the celestial vault arching across the night sky. Even in the earliest-known texts of various ancient cultures, the stars were commonly grouped into anthropic figures which were believed to represent divine persons, sacred animals, and objects of religious importance. Crater was originally The Chalice, for example. All of the major cultures in the Near East, the Far Eastern lands, and the New World developed an indigenous system of constellation figures and associated legends which were graven on stone, recorded in written texts, depicted on maps or on globes. Of these various constellation systems, the Sumerian/Babylonian system of constellation figures endured to become the most influential. Improbable and enigmatic as figures like Sagittarius, Capricornus, and Pisces may be, as art in the sky these constellation identities lived through adoptions, adaptations, aberrations, and additions by Greek, Roman, Islamic, and European astronomers, even as astronomy evolved into the disembodied mathematical coordinate systems of today. The only place astronomical objects are not located by constellation name is in the professional literature. Up to the middle of the 19th century, celestial atlases and globes were considered to be art first and precision later. Nearly all historical sky atlases were collaborations of astronomers with artisans. Only the artisans, after all, could engrave on copper plates in reverse, and in intaglio. The astronomers sketched the positions of the stars and the constellation names; the artists took care of the presentation. The result was as much art history as astronomy history. There are so many collections of celestial atlases that Nightfall could devote an entire issue to them. Since our goal is the deep-sky today and not in history, we rest content to open the door to the past just a crack for a peek inside. The rest is up to you and the good graces of Wiki and Google. “Celestial cartography” (1, 2) is a good place to start.

Hydra, Crater, and Corvus from Zīj-i Sultānī, an astronomical atlas published by Ulugh Beg of Samarkand in 1437. As with Western cartography, it was a collaboration between artists and astronomers. Unlike the West, where the artists sometimes were rather more liberal than the astronomers, Ulugh Beg determined the length of the tropical year as 365d 5h 49m 15s (an error of +25s and more accurate than Nicolaus Copernicus' error of +30s). Beg also determined the Earth's axial tilt as 23.5047 degrees. Hydra faces the opposite direction in the sky than as seen from Earth because this is the view Allah would have upon looking down from Paradise through the stars to Earth.

HAS a long lineage Haemisphaerium Stellatum Australe (Southern Hemisphere Constellations) published in 1660 by Andreas Cellarius depicted southern skies superimposed over North and part of South America. The chart is centred on the ecliptic pole 20 degrees into the northern hemisphere. The projection is as if gazing from deep space down through the constellations figures onto the terrestrial world below. Hence the constellations face in the opposite direction from the way we see them looking up from earth. The unusual projection originated in Petrus Plancius's theory that the stars remained in a sphere-like configuration above the earth and moved in coordination with the earth. Andreas Cellarius was born in 1596 in Heidelberg but emigrated to Holland in the early 17th Century Cellarius' best known work is his Harmonia Macrocosmica, first issued in 1660, by Jan Jansson as a supplement to Jansson's Atlas Novus. Cellarius' charts are the most sought after of celestial charts, blending the striking â&#x20AC;&#x201C; some say florid â&#x20AC;&#x201C; imagery of the golden age of Dutch Cartography with contemporary scientific knowledge. The original was 50.8 x 43.8 cm (20 x 17 in) and coloured by hand using vegetable tints. (Tints are transparent; dyes are opaque.) Source: Barry Lawrence Ruderman Antique Maps.

Albrecht Dürer’s 1515 maps of constellations from the Northern and Southern Hemispheres combined the most up to date scientific knowledge of the stars in his time with Dürer’s pioneering skills as a woodcut illustrator. He created his pair of charts with the cartographer Johannes Stabius, based on the work of astrono-mer Conrad Heinfogel. The 48 Northern constellations named by Ptolemy in the 2nd century appear, as do the 12 Zodiacal signs. Dürer’ss Southern chart depicted the complete Argo Navis, the Greek ship of Jason and the Argonauts, ploughing through the Southern seas in the sky. Today the old Argo Navis has been turned into Vela, Carina, Puppis, and Pyxis. The rest of the Southern Hemisphere is empty because it hadn’t yet been plotted by Europeans. Throughout his career, Dürer viewed the mass production and commercial enterprise as the engine of what we call the information industry. Dürer intuitively understood that a permanently recorded image was a better way convey information than mere text. Dürer wasn’t the first in the world to print a star chart — astronomer Su Song’s Xin yixiang fayao was printed in China in 1090. But Dürer was the first to combine existing factual knowledge with captivating visualisations that would endure permanently. Even today, five centuries later, we can readily recognise constellations in the sky exactly as they are depicted on Dürer’s maps.

This untitled celestial map of the southern skies was issued in 1674 by Ignace Gastone Pardies in Paris. Pardies (1636-1673), was a Jesuit astronomer whose manuscript star atlas shown here was published by his colleague Thomas Gouye just after Pardies’ untimely death at the age of 37. The map is one of six in Globi Coelestis Tabulas Planas Redacti descriptio ... opus postumum, with engravings made by G. Vallet. The vivid renderings of the constellation figures, e.g., Indus and Toucan, and nowobsolete constellations like the unnamed tree between Volans and Centaurus’s tail, were modified from the depictions invented by Johannes Beyer in his landmark 1604 Uranometria. Gouye's 1688 Observations Physiques et Mathematiques that included Pardies’ maps was an important scientific work published by the Jesuits, a Catholic religious order. Gouye learned the lore and configuration of the southern skies during his embassy to Siam in 1685, which included a layover at the Cape. Gouye’s astronomical observations were noteworthy for being true to science rather than the rather travelogue-like air of terrestrial cartography in that era. Tables of astronomical data, instrumentation, and weather conditions, and astronomical positions (not shown here) were the equivalent of today’s vast astronomical legacy data in computers servers at universities and observatories. Look closely near the celestial pole in the lower left quadrant, where Magellan’s clouds are shown as “Nebecula Major” and “Nebecula Minor”. Map source: Bibliothèque Nationale, Paris.

How did they make these maps? were engraving hammer-forged iron to decorate suits of armour. Musical At one time there were only three ways to reproduce reality in an enduring instruments and religious icons soon followed. physical form. The earliest known was children; this method had a high easeIn one of history’s more felicitous coincidences, ten years before of-use factor and readily lent itself to the learn-as-you-go school of pedagogy. Gutenberg’s first Bible an artist named Martin Schöngauer experimented The other two methods were (a) additive: applying material to a supporting with engraving fine lines into soft copper sheets. While Gutenberg’s transfer surface, usually as paint, ink, or mosaic; and (b) subtractive: removing material process used inked letters raised above an un-inked base, Schongauer’s from a substrate, producing bas-relief or in-the-round sculpture. process was just the opposite: ink was applied to the surface of a flat copper One of the most difficult objects to reproduce accurately was the shape sheet, filling tiny grooves incised into the copper. The excess surface ink was used to represent a sound, i.e., a letter. Words are easy to make with ink and a wiped away with starched cheesecloth, leaving only the ink in the grooves. quill, difficult and time-consuming with a chisel. With the introduction of When the plate was run through paper into Spain in 1056 and a press, the paper absorbed the manufacture in France in 1190 ink in the grooves, reproducing words could be mass produced. the image reversed from side to With Johannes Gutenberg’s side but otherwise accurate. Bibles in 1440 words on paper The net effect was a much finer, became viable commercially. more nuanced line. This new There was little market for form of image transfer came to maps and charts until the boom be called intaglio (“cut into”). in maritime exploration in the fifteenth century. India was hard Science’s first multitask to reach if you didn’t know where it was. The need for maps medium The transition from suit of armour to a map of the constellations reflects the wealth of the ultimate user inspired a technology for making There was a bit of prior as much as it does the lateral shift of a technology from one-off exotic to mass produced consumable. them. Engraving on metal plates technology to the use of copper was the ideal solution but most plates. Decades before Martin Schöngauer, medieval bookbinders stamped maps were navigational; information for its own sake was a late bloomer. gold-leaf letters into leather and parchment covered manuscript books. They Only in 1515 was the first printed celestial chart of any consequence made, a used brass dies with protruding Latin letters. Struck with a fierce blow from a pair of maps by Albrecht Dürer depicting the northern and southern skies. hammer the die compressed the gold leaf deeply into the leather. It was Images graven on metal have a rather brief history as artistic matters go. moveable type one letter at a time. Antiquarians and numismatists still use The first images made by the transfer method (making a duplicate using an the term “strike” to describe decorative stamping a “blank” of paper or coin. original) was the woodcut. Early attempts at image fidelity using the medium Martin Schöngauer was the first to hit on the idea of copper sheets as an of wood were a net loss to the ideals of beauty. imaging medium. The first words and illustrations to be depicted were Goldsmiths and ironmongers were the first to take advantage of the fact neither saints nor hellions, but lowly playing cards. It’s a miracle any that wood does not easily aspire to elegance, but metal does so readily. The survived, but about 100 still exist. The discovery of a new and unknown earliest notably attractive images on metal to come out of the Middle Ages 1430s playing card is cause for champagne toasts in the rarefied circles that were not monastic Bibles, but aristocrats’ weaponry. By 1400 goldsmiths were can afford to pay exotic sums for bits of old paper. engraving hammer-forged iron to decorate suits of armour. Metalsmiths 66

Quick to adapt were the prolific stampers of saintly medallions that rose out of the pilgrimage industry (for so it was). One of them was Herr Gutenberg. He printed locket-sized images of the Virgin & Child long before he devised moveable letters. A 1440 engraved image of the Passion of Christ on a small copper plate still has traces of ink in the grooves. The modest copper plates used to produce playing cards were speedily enlarged into folio-size sheets that could be fit together to imprint 8 separate pages on a sheet of standard-size rag-mill paper. The first copperplate maps were two Italian editions of the geographer Ptolemy struck in 1472. Copper was far superior to wood because of the much finer detail that could be graven into a polished copper surface. Most maps of the time were sold to the maritime trade as navigational aids. For the first time sea captains could compare their relative latitude on the sea with the landfall they were heading for. Close to shore, however, a wrongly plotted map could end in shipwreck. Lives and ships could be lost if Elephantine has real meaning in the antiquarian underwater ledges or book world. All the more so in the above example, treacherous straits were which is the largest size paper used for printing, the inaccurately sounded. Hence double-elephantine folio. The word “folio” simply every sea captain ordered means “sheet”. plumb-bob soundings to be made (remember “full fathom five”?) as they neared the shore. They jotted changes to shoals on their charts, to be handed over to their home-port mapmakers when they returned. Numerous examples of scribbled resoundings can be found in the archival charts of a maritime museum. But since their corrections involved only a small portion of a large chart, re-engraving the entire plate was infeasible as too expensive. Copper was

ideal for small-scale corrections. The map engraver simply flattened the old grooves with a smooth stone and incised new ones, numbers and all. Little did those mapmakers ever dream that centuries later entire master’s theses would be produced tracking the exact sequence of undated corrections. Oceanographers today use the chronology of alterations of old maps to plot long-term current patterns in a given water body across time spans of a century. Geographers in turn have traced the changes in coastlines as a tool to understand more general thinking of respective eras. The French king Louis XIV made geographic history when he remarked upon first seeing Cassini’s accurately replotted French coastline, “Why sir, you have robbed a sixth of my country from me.” Cassini saved himself from a bad moment by responding, “No, Your Majesty, the ocean did.” __________ Figure on overside: Amsterdam or Antwerp canal-side strollers would not have been surprised to pass the open doors of a print-making establishment like this. Dim, dank from damp paper and sweat, fetid from wood fire smoke in the winter. The shop urinal was a pile of wood shavings in a corner. Today these are called job shops. As then, they produce a small but vital component in a much larger work. It was easy to re-set a typo in the age of letter-by-letter typesetting; difficult and expensive to correct or update a harbour fathom chart in a nautical chart where, halfway around the world a year later an accidentally inverted 6 that read 9 could turn a proud ship into a catastrophic loss of all hands. The 16th century’s great age of global exploration to fuel mercantilist economic ambitions spawned a myriad small trades like copperplate printing and sextant engraving. One modest corner of this burgeoning world of enterprise produced the works that so mesmerise us today: the cartographical plate. They were a mix of high art and high precision in a merger science begrudges no more. The modest folio-sized plates being prepared in this little drukkerij would have appeared in leather-bound books small enough to be read in one’s lap. Cartographical plates, though, were the largest objects printed at the time. The aptly named elephantine folio 23 inches ( 58.4 cm) tall was surpassed only by the double elephantine folio at a colossal 50 in ( 127 cm) tall – eye-level or higher to most of the men working in this shop. It is also to be remarked that child labour was called ‘apprenticeship’ and lasted longer than grammar school today.

Copperplate printing in 16th century Amsterdam

Celestial Cartography on the Web • Atlas Coelestis: Comprehensive website by Felice Stoppa with digital editions of various celestial atlases and maps from the 15th to early 20th century. • Atlas Coelestis: Collection of celestial atlases in the Biblioteca “Guido Horn d’Arturo” del Dipartimento di Astronomia dell’Università di Bologna. • The Mercator Globes at Harvard Map Collection: 1541 terrestrial & 1551 celestial globe of Gerard Mercator in the Harvard Map Collection. • Tycho Brahe (1546 – 1601): The Astronomiæ instauratæ mechanica (1602) digital copy of the instruments used by Tycho Brahe. • Andreas Cellarius (c. 1596 – 1665): Harmonia Macrocosmica (1660, 1661, 1708). • Vincenzo Coronelli, (1650 – 1718) 110-cm celestial globe (1693). • Johann Gabriel Doppelmayr (1677 – 1750): Atlas Coelestis (1742). • John Flamsteed (1646 – 1719): Atlas céleste de Flamstéed (1776 Paris ed.). • Johann Elert Bode (1747 – 1826): Vorstellung der Gestirne auf XXXIV Tafeln (1782). • Franz Niklaus König (1765 – 1832) Atlas Céleste (1826) printed in reverse white upon black. Municipal and University Library of Berne. • The Earth & the Heavens:The Art of the Mapmaker. Collection of the British Library (London). • Ciel & Terre: Based on the exhibition Figures du ciel held at the Bibliothèque nationale de France (Paris). • The Earth & the Heavens:The Art of the Mapmaker. Collection of the British Library (London). • Linda Hall Library of Science, Engineering & Technology. Digital online editions of numerous historical star atlases (click on ‘COLLECTIONS’). • Dunhuang Star Chart, the Oldest Sky Atlas in the World, International Dunhuang Project, British Library • Tycho Brahe’s first quadrant built in 1576 or 1577 was made of brass mounted on masonry pillars to give him foot room beneath so he could make measurements while seated. The vertical and horizontal arcs were 65 centimetres in radius. Its estimated accuracy was 48.8 seconds of arc in both azimuth and declination. It was one of the first instruments built at Hven, a small island that was to evolve into his famed Uraniborg Observatory. It was completed in time to observe and measure the path of a comet that appeared in 1577. The altitude axis of the quadrant rotated around the finelygraduated base circle, Brahe’s positions for an object had to be labourously converted from alt-az to ecliptic coordinates. Brahe deemed the mathematics of this to be easier than to mount such a ponderous apparatus at the 55.9° angle needed to align with the Earth’s rotational axis.

Mr. Herschel does his homework William Herschel began keeping his Commonplace book on Astronomy in 1759, when he was in his early 20s, and its entries date into the 1770s. The 104 handwritten pages trace Herschel's work and thought while he learned to build his first telescopes. These would allow him to see farther into the heavens than humans had seen before. He discovered the planet Uranus in 1781 and both studied and speculated on nebulae and the evolution of the stars. Images courtesy of Linda Hall Library, Kansas City, MO, USA.

When Herschel wanted to pin down the orbital diameters of each of Jupiter’s satellites, he calculated the parallax tangents of the position of the satellite at first or last contact with Jupiter’s disc. The firstcontact time difference introduced by the diameter of the Earth’s orbit are shown for each satellite designated as 1, 2, 3, 4.

The orthography of Saturn’s rings with respect to the ecliptic during one-half of a complete orbital cycle.

Calculating the position of an object moving across the disc of Jupiter during one 9 hr 56 min rotation (total 596 minutes). In 10 minutes the object moves 6° 2’ 40” across the disc. Across the next two pages Herschel calculated the slowdown rate as the point receded toward the edge and out of view.

Toward the end of the notebook, in 1772 Herschel endeavoured to calculate the differences between spherical aberration in a simple concave spheroid compared with the aberrations from a paraboloidal figure. Unfortunately he was unable to deal with the paraboloid’s perennial nemesis, coma.

The Gamma Velorum Cluster

Source: Paulo Cacella, BrazĂia, Brazil

Colliding-wind binaries raise hell in the heavens and in the process teach us how to make better textbooks The colliding winds produced by ultra-massive stars are the fiercest tornados Alas for all their crash-diet efforts, Wolf-Rayets are doomed no matter how a star cluster can make. They are so hot they radiate mainly in X-rays, which is hard they try to lose weight. They are behind on the ejection-vs-evolution why the beautiful multiple star Gamma Velorum (γ Vel ) seems to us a serene curve and will expire as core-collapse supernovae, not because they may or and lonely quadruplet in the midst of a pretty, speckled star field. If we were may not get below the supernova minimum-mass threshold of ~8 M⦿, but closer — say gazing upon the scene from from the balcony because the nuclear reactions in their cores have advanced seats of Jupiter’s moons — we would see the region beyond the ability of their envelope mass to constrain. The presently occupied by our Earth to the Sun as a blindingly typical evolutionary pathway for such stars is O supergiant hot maelstrom writhing like the most furious coronal flares becomes a luminous blue variable (e.g., η Carinae) ➞ Wolfshown in videos of our Sun, shivering the entire sphere Rayet (nitrogen N or carbon C type depending on mass-towhere our Sun-Earth diameter would be. We would need luminosity ratio at the time) ➞ core-collapse SN1c (possibly X-ray and UV glasses to see this, though, because little light gamma ray burst) ➞ black hole. No two ways about it, if in our familiar visual band would reach us. you are fated by your birth girth to become a Wolf-Rayet, Astronomers have identified two-dozen γ Vel-like your future career prospects are pretty awful. multiple O-class supergiant systems in our Galaxy. γ Vel is Next to γ2 is the bright mag, 4.3 γ1 Vel, a blue-white one of them. To us, a pretty eyepiece of four brilliant stars subgiant B star. γ1 lies 41.2 arcsec from the Wolf-Rayet in a truncated-arrow shape, a closer look using UV and XHand-sketched colliding-wind image from binary, easily resolved with binoculars. The multiple system ray spectroscopy shows it to be a six-star multiple whose Feldmeier & Shlosman 2001 based on Lucy has several fainter companions as well: γ Vel C, a white A & White ApJ 1980. combined luminosity make it the brightest star in Vela. The star of mag. 8.5 some 62.3 arcsec from γ2, and the binary pair most luminous component γ2 Velorum is a spectroscopic γ Vel D and E about 93.5 arcsec away. The system is visible from Earth south of binary consisting of a massive blue supergiant and the closest known Wolf35°N latitude. Rayet star to Earth. They orbit each other in 78.5 days in an oblate ellipse γ Vel has a traditional Arabic name, Suhail al Muhlif, and the modern name whose a mean separation is 1 AU, the distance of Earth from the Sun. γ Vel’s Regor. Neither name has been approved by the International Astronomical Wolf-Rayet is the nearest supernova candidate to Earth. Union. Too bad, because the ancient Arabic translates as, “the glorious star of The hot evolved O star is about 25 times as massive than the Sun (25 M⦿) the oath.” with a surface temperature of 35,000 K, making it 200,000 times more __________ luminous thank the Sun (2 x 105 L⦿). Its Wolf-Rayet partner is 50,000 L⦿ and a Further reading: What is Gamma Velorum? surface temperature of 60,000 K. This star, like other Wolf-Rayet stars, started On the binary nature of massive blue hypergiants out with a mass of about 40 times that of the Sun but has shed much of that Self-regulated shocks in massive binary systems original material; it is only about 10 M⦿ today.

Where do the X-rays come from? Gas from O-star surfaces erupt brilliantly in X-rays. Add this to the fearsome UV radiation produced by the stars themselves at their surfaces, plus the fact

The relatively close proximity of γ2 Vel creates high-energy colliding stellar winds as the two stars orbit each other. A recent study of gamma-ray energies

that the winds are driven from the stars at 1000 to 4000 km s-1, it is hard to imagine an environment more inimical. A fraction of the wind’s energy is driven by radiation pressure (heat transfer when photons hit dust); some comes from the kinetic pressure of ionised protons colliding with heavier ions such as carbon, nitrogen, oxygen, etc.; and some comes from momentum transfer (which differs from energy transfer) as high-speed helium ions strike the more massive metal ions in the heat-shocked gas. X-rays are generated mainly during the helium particle/metal ion energy exchange.

from seven Wolf-Rayet systems in the Milky Way determined that γ2 Vel is the most energetic gamma, X-ray, and UV emitter. The only other gammaemitting stellar system in the Milky Way is the Eta Carinae system. However, where η Carinae lies in a seething bath of shocked gas and magnetic fields, γ2 Vel shows no discernible nebulosity in the 2MASS optical, XMM Newton, or AKARI 140, 65, and 30 µm bands as far out as a 30 arcmin radius from the cluster. With this stellar content, the Vel system is oddly bereft of formation ejecta. The dearth of nebulosity is all the more remarkable because Vel is one of the most fulminous colliding-wind binary systems in the sky. There should be scads of gas flying all over the place. Where is it? One possibility is that the normally abundant dust that is found in a starforming nebula is simply incinerated by the incredibly hot environment around a system as massive and hot as γ2 Vel. All by itself, the γ2 Vel binary puts out more light in ten minutes than our Sun generates in an entire year. Their winds are so powerful that they eject an Earth's worth of material roughly once every month. Imagine those two winds as colliding head-on. Wind collisions of any kind generate a lot of heat, and therefore light, but the γ2 Vel system produces gas temperatures on the order of 4 to 10 million K. Galactic dust particles begin to boil away their atoms at about 6000 K, thousands of times cooler than the gas around γ2 Vel.

X-rays are generated at the end-stage of a chain of ablation/reverse shock dissipations in which some X-rays are re-absorbed, heating the gas, while other X-rays escape, cooling the gas. Shock front deformation is strongest at the edges of the front while X-ray emission is strongest hear the central peak. Source: Hydrodynamics of astrophysical winds. This alternative colliding-wind model is provided by courtesy of the Flat Earth Society.

The difference between subsonic (l) and supersonic (r) colliding winds.

If it’s a-blowin’ in the wind, which way is the wind blowing ?

Simulation of particle driven wind from an accretion disk, looking down along the star’s polar axis. Click this link to see the full animated gif of the wind in action.

The fearsome hydromagnetic winds which originate in the envelopes of massive hot stars are ejected directly away (radially) from the star disc at hypersonic velocities. They also carry a rotational component from the equatorial surface rotation of their stars. Moreover, the superheated gas at the surface of the star is not constant and even. The relatively shallow convection zones of massive hot stars churn and boil with gigantic bubbles that pop when they reach the surface. They hurl varying amounts of superheated interior gas into the stars’ corona, where the gas enters the stars’ ejection disc. There the gases are spun up by the shear torque of the whirling disc. Since the gases are so highly ionised, they acquire an additional magnetic vector from the magnetic fields of the disc, which aligns not with the equatorial plane of the disc but rather the star’s magnetic polar axis. To make life even more complicated, the escaping gas bubbles flow at one speed while the disc is rotating at another. An interface between two gas flows quickly develops small wavelets, then large waves, and finally giant turbulence cells. You can watch this as a breeze crosses a still pond, setting up rows of ripples that grow in size as the wind

Significant variations from the basic Wolf-Rayet/O supergiant colliding-wind model occur because of the wide disparity of star types involved and the proximity of nearby stars.

blows along the “fetch” of the total water expanse. In gas bodies, differential wind shears set up a looping set of wave-like perturbations called KelvinHelmholtz instability. We are familiar with K-H instabilities right here on Earth when we watch videos of a flat cloud layer erupt into large gyres as the layer blows past a mountain peak or island below. (See 1, 2, 3.) The superheated gases are so hot (>10,000 K) they are entirely ionised. That means a lot of electrons (flux density) moving at high speed (flux velocity). They are easily channeled by the already-existing poloidal magnetic field of the stars themselves. A poloidal magnetic field is openended along the the polar axis so it takes on an “hourglass” configuration (see image next page). This geometry evolves because stellar gas expulsion flows along magnetic field lines readily but strongly resists flowing across them. The overall result is a deeply curved flow field that looks rather like a bow shock. Unlike bow shocks, these flows become turbulent.

Why do so many colliding winds end up looking like hourglasses? The flux threading the molecular disk acquires an hourglass shape due to preferential contraction of gas along magnetic field lines. However, real systems don’t always follow idealised models such as the model below. Most studies show that outflow and magnetic field axes are aligned to within only withint ±20° of the disc plane. When a star’s rotational axis and magnetic field axis are offset (as our Earth’s is) the gas flow adds a spin vector which corkscrews the wind. Astronomers have a major job untangling the spectral signatures of colliding-wind binaries since we are looking through an optical straw passing through a complex set of flow paths, shock fronts, and magnetic field lines. The lines are blueSource: Pudritz & Norman shifted toward us in their approach

Hydrodynamical simulations of the colliding winds in the binary system WR 22 when the pair are at apastron (furthest apart). Temperature differences in the absorption-wind band of expelled gas move the Kelvin-Helmholtz frontal surface from the inner to outer edge of the ablation surface. X-rays are generated in the black-tinted portions of the absorption wind. Source: Gosset, Nazé et al 2009.

direction and redshifted where they recede. Moreover, γ2 Vel’s Wolf-Rayet ejection bubble spectral lines are broad and tend to overwhelm weaker lines from the O supergiant companion. The situation is even worse if the star is a

Three-D astronomy simulations of star cluster evolution are a veritable cottage industry these says. There’s even a DIY recipe book. Here is a sampler: Matthew Bate, Mark Krumholz; Max Planck Institute; SILCC Project; Auriga Project; Chris Federrath; J-P Metsävainio; Jim Stone, Bouy & Alves; Pelupessy, CLUES, Daniel Pomarède & Hélène Courtois. Plus these.

close binary with two very different stellar signatures, as is the case with γ2 Vel’s poor choice of bed partners. They’re an odd couple veering between amusing and disastrous. Lucky us that we have back-row seats!

Source: NASA image composite of Chandra, Spitzer, and Hubble Telescope images.

Shock fronts on scales as small as a supernova blast wave and as large as a galaxy cluster are prolific star producers. Near the centre of the Milky Way three successively larger superbubbles from the massive young clusters Quintuplet and Arches, and Central expand outward, compressing gas into smaller clusters as they enlarge but also weaken.

The bright edges can be in the millions of Kelvins and several light years thick at their highest densities. A less known side effect of these kinetic temperatures is the dissolution of bound atoms into ions, which reassemble into new and more complex chemical molecules as they cool. The new molecules become the soup that makes planets like Earth.

Source:!Antonio,Vargas

The Great Orion Nebula By,invitation,to,our,friends,across,the,pond:,Andrew,James,from,his,Southern,Astronomy,Delights

Early History of the Orion Nebula The telescopic history of the Orion Nebula and the M43 region is quite an interesting story and contains one of the greatest conundrums in astronomy, worthy of a decent detective mystery. First realised by Alexander von Humboldt, none of the observers before Galileo had noted the particularly bright nebulosity, although to the naked-eye today, it is obviously bright and definitely not stellar. Why this was not observed has yet to be properly explained, meaning either that the Orion Nebula was simply not noted or the seemingly highly unlikely possibility that the nebula was not as bright in the time of Galileo. More strange is that nobody else prior to him has written down the visibility of this particularly famous ‘Grand Nebula’ of the entire sky. The first written deep-sky observations, like those of Abd alRahman Al Sufi or Azophi, who in 986 A.D. published his “Kitab suwar al-kawakib” or “Book of Fixed Stars”, were highly detailed, He even noticed for the first time the extragalactic spiral galaxy — Andromeda Nebula (M31). Yet he does not mention or show any nebulosity within Sword of Orion. M42 is obviously not a star, even with the naked eye, though one explanation of this discrepancy nebula may not have been as bright during this time. By 1603, Johann Bayer in his Uranometria made his highly detailed star chart of the region, and this also does not show any nebulosity within the Sword of Orion. Yet, Bayer had noted the two stars in the Sword as an optical double, and named them as Theta (θ1) Orionis and Theta (θ2) Orionis.

Orion!in!a!1280!edition!of!Rahman!al4Suﬁ’s!! Book,of,Fixed,Stars,!written,c.!986!ACE.

Amazingly, the renown Italian scientist and astronomer Galileo Galilei never mentions it, even though it was one of the first areas for his telescopic sweeps. In December 1610, he extensively observed the constellation of Orion, merely noting an increase in the number of stars in the region. (See Fig. 2 prev. page) Added were an additional eighty stars or so within the sword, which Galileo made in the subsequent drawn figure in his Sidereus Nuncius or The Sidereal Messenger. Nicholas Peiresc in November 26th, 1611 made the first known telescopic observations, which continued between 6th to 10th December. Written observations of Peiresc states he was ‘surprised’ to find ‘a small illuminated cloud’. This was followed by Johann Baptist Cysat (Cysatus), a Swiss Jesuit priest, made the next known observation in 1618, while comparing the nebulae with a nearby comet visible at the time. Christian Huygens gave the first detailed description using a Galilean telescope in 1656. Huygens thought he had discovered M42 for the first time, and subsequently published observations in his Systema Saturnium, a few months after the original observation. It contained his full description and drawing, and told of the remarkable star Theta (One) Orionis (θ1 Ori) in the very heart of the nebula. His original sketch, contains seven stars inside the nebula, with five lying just beyond it. The stars of the Trapezium, he curiously only notes as three stars, though for the telescope aperture used, he should have easily seen all four. For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately oﬀer themselves to view, of which very many appear rather large and very conspicuous but the multitude of small ones is truly unfathomable.” Galileo, Sidereus Nuncius

Galileo’s!Telescopic!Drawing!of!Orion,!with!our!annotations.!! The!ﬁgure!shows!the!whole!asterism!of!Venus’s,Mirror!—! including!Orion’s!Belt!and!Sword.!This!drawing!clearly!shows! the!missing!Orion!Nebula,!which!should!have!been!easily! seen!by!the!naked4eye!but!was!somehow!missed!or!was! simply!not!annotated.

The brightest stars within the nebula were noted early and cataloged as one bright star of about fifth magnitude: In about 130 AD, Ptolemy included it in his catalog, as did Tycho Brahe in the late 16th century, and Johann Bayer in 1603 – the latter cataloging it as Theta Orion in his Uranometria. In 1610, Galileo detected a number of faint stars when first looking at this region with his telescope, but didn’t note the nebula. Some years later, on February 4, 1617, Galileo took a closer look at the main star, Theta1, and found it to be triple, at his magnification of 27 or 28x, again not perceiving the nebula.

Later History and General Observations of the Orion Nebula Perhaps the first detailed descriptions of the Orion Nebula was by the Sicilian amateur scientist Giovanno Batista Hordierna (1597-1660), whose observations have only recently came to light in 1985. In his small catalogue of forty objects, he list and drew the Orion nebulae as No.5. Hordierna describes in his 1654 “De systemate orbis cometici; deque admirandis coeli characteribus” [Systematics of the World of Comets, and on the Admirable Objects of the Sky.]

“…in the sword of Orion and includes 22 stars as one can see with the telescope. But this Luminosa is more admirable because of some unresolvable luminosity in whose middle can be seen three stars.” Another description of importance appear by Edmond Halley in Philosophical Transactions “An Account of Several Nebulae or Lucid Spots Like Clouds, lately Discovered Among the Fixt Stars by Help of the Telescope.” (Phil.Trans., XXIV, pg.390-391 (1716); here Halley (here partly translated) describes;

“The first and most considerable is that in the Middle of Orion’s Sword, marked with Theta by Bayer in his Uranometria, as a single Star of third Magnitude; and is so accounted by Ptolemy, Tycho Brahe and Hevelius: but is in reality two very contiguous Stars environed with a very large transparent bright Spot, through which they appear with several others. These are curiously described by Hugenius [Christiaan Huygens] in his Systema Saturnium page. 8, who there calls this brightness Portentum, cui certe simile aliud nusquam apud reliquas Fixas potuit animadvertere [a wonderful object, which is certainly unique among the fixed stars]: aﬀirming that he found it by chance in the Year 1656. The Middle of this is at present at

A later star plot in Galileo’s Sidereus Nuncius shows a “Nebulosa Orionis", but upon closer inspection the star patterns don’t match the Orion Nebula’s stars. The middle paragraph in the text above makes it clear that Galileo was referring instead to the asterism which we call Lambda Orionis, the head of Orion.

In March 1769, Charles Messier placed Orion’s Great Nebula in his famous catalogue of 103 objects; as M42 and M43. He separated the nebula into two parts because he thought they were different objects, and even made a drawing of them. Messier described the Orion Nebula region in the first version of the Messier Catalogue in the French Mem. Acad. for 1771, pg. 450-451 (1772), just two years prior to William Herschel in 1774; “I have examined a large number of times the nebula in the sword of Orion, which

Huygens discovered in the year 1656, & of which he has given a drawing in the work which he has published in 1659, under the title Systema Saturnium. It has been observed since by different Astronomers. M. Derham, in a Memoir printed in the Philosophical Transactions, no. 428, page 70, speaks of that nebula which he has examined with a reflecting telescope of 8 feet [fl]. Here is the translation of what he has reported in this Memoir.” “…only that in Orion, hath some Stars in it, visible only with the Telescope, but by no Means sufficient to cause the Light of the Nebulose there. But by these Stars it was, that I first perceived the Distance of the Nebulosae to be greater than that of the Fix’d Stars, and put me upon enquiring into the rest of them. Every one of which I could very visibly, and plainly discern, to be at immense Distance beyond the Fix’d Stars near them, whether visible to the naked Eye, or Telescopick only; they seemed to be as far beyond the Fix’d Stars, as any of those Stars are from Earth.” “M. le Gentil also examined this nebula with ordinary refractors of 8, of 15 & of 18 feet length; as well as a Gregorian telescope of 6 feet, which belongs to Mr. Pingré. He has published his observations in a Memoir which can be found printed in the Volumes of the Academy, year 1759, page 453. There is a joint of the drawings which he had made of it at that time, as well as those of Huygens & of Picard; these drawings differ from each other, so that one may suspect that this nebula is subject to sort of variations. Here is what I have reported about that nebula in the Journal of my Observations. On March 4, 1769, the sky was perfectly serene, Orion was going to pass the meridian, I have directed to the nebula of this constellation a Gregorian telescope of 30 pouces [83cm.] focal length, which magnified 104 times; one saw it perfectly well, & I drawed the extension of the nebula, which I compared consequently to the drawings which M. le Gentil has given of it, I found some differences.

“This nebula contains eleven stars; there are four near its middle, of diﬀerent magnitudes & strongly compressed to each other; they are of an extraordinary brilliance: here is the position of the brightest of the four stars, which Flamsteed, in his catalog, designated by the greek letter Theta, of fourth magnitude, 80° 59′ 40″ in right ascension, & 5d 34′ 6″ in southern declination : this position has been deduced from that which Flamsteed has given in his catalog.”

In March 1774, Sir William Herschel observed M42, and later in the successive years of 1801, 1806 and 1810. His theory, written in The Nebula Hypothesis, realising for the first time the true nature of the Orion nebula. He wrote on M42;

It is possible that these stars may have switched to a more energetic process, or that the star has suddenly ejected the incipient nebulosity that surrounds them. This may be the case with other stars observed in this region. Observation using infra-red wavelengths and the Hubble Space Telescope (1994-95) have revealed many observational details of the formation and cause of such brightening.

“…which would see here an unformed fiery mist, the chaotic material of future suns.” Noting that the observations made by him were different from Huygens, William Herschel stated in December 1810 the belief that the nebula had actually changed. Some thirty-seven years later, he was absolutely convinced that several stars had actually dissipated their nebulosities. By comparing his sketches, he also noted changes in the appearance of the nebula’s internal structures. It seems, by our knowledge of the meticulous detail of his observations, it is likely that these changes were real. Later other were to question the same thing about other nebulae like Eta (η) Carinae.

In 1826 Sir John Herschel made observations whose descriptions remain as the standard, and even today are still quoted in most of the astronomical texts. His most poignant description was written in 1837, and says;

“I know not how to describe it better than by comparing it to a curdling liquid, or to the breaking up of a mackerel sky where the clouds of which it consists beginning to assume a cirrus appearance.” Reverend Thomas Webb in his “Celestial Objects for Common Telescopes.” (1859)

Compared to modern observations, several prominent and obvious stars of the Trapezium are missing. i.e. The ‘E’ and ‘F’ stars. Wilhelm Struve discovered the fifth ‘E’ star of the Trapezium in 1826. His magnitude estimations, show it has brightened considerably in the last 180 years. Some have speculated that many of these stars are very very young. The 110 stars discovered in the inner portions of the nebulae each show embryonic material, and proto-planetary disks which are being ‘shredded’ over a short time. In 1936, observations the variable star FU Orionis was found to have brightened by at least six magnitudes over a short period of several months! Ever since this star shows small fluctuations in brightness but has remained at an elevated average of 10.3 magnitude. Another two fainter variables of V1057 Cyg and V1517 Cyg also have shown similar phenomena.

“The Great Nebula, one of the most wonderful objects in the Heavens;…” Richard A. Proctor also in the late 1870s, described it as follows:

“…the thought that seemed so impressive, so thrilling, as to surpass even the feeling of awe with which is the solemn darkness of night we see some mighty group of suns sweep into the field of view of the telescope… that here on this tiny square inch of shoreline, with its thin film of chemical sands, has been traversing the solemn depths of space… Here we have mirrored by Nature herself that marvellous round of milky light below* Orion…” (* as observed from the northern hemisphere.)

Several years later Proctor again wrote, in his charming; “Half-Hours With the Telescope : Being a popular guide to the use of the telescope as a means of amusement and instruction”, in which he beautifully describes the finding and observing of the Orion Nebula as;

far larger probably than those of the whole system whereof our sun is a member. I believe this view is founded on insuﬀicient evidence, but this would not be the place to discuss the subject. I shall merely point out that the nebula occurs in a region rich in stars, and if it is not, like the great nebula in Argo, clustered around a remarkable star, it is found associated in a manner which I cannot look upon as accidental with a set of smallmagnitude stars, and notably with the trapezium which surrounds that very remarkable black gap within the nebula. The fact that the nebula shares the proper motion of the trapezium appears inexplicable if the nebula is really far out in space beyond the trapezium. A very small proper motion of the trapezium (alone) would long since have destroyed the remarkable agreement in the position of the dark gap and the trapezium which has been noticed for so many years. But whether belonging to our system or far beyond it, the great nebula must have enormous dimensions. A vast gaseous system it is, sustained by what arrangements or forces we cannot tell, nor can we know what purposes it subserves. Mr. Huggins’ discovery that comets have gaseous nuclei, (so far as the two he has yet examined show) may suggest the speculation that in the Orion nebula we see a vast system of comets travelling in extensive orbits around nuclear stars, and so slowly as to exhibit for long intervals of time an unchanged figure. “But of such speculations” we may say with Sir J. Herschel “there is no end.” To return to our telescopic observations: The trapezium aﬀords a useful test for the light-gathering power of the telescope. Large instruments exhibit nine stars. But our observer may be well satisfied with his instrument and his eye-sight if he can see five with a 3½-inch aperture. A good 3-inch glass shows four distinctly. But with smaller apertures only three are visible. The whole neighbourhood of the great nebula will well repay research. The observer may sweep over it carefully on any dark night with profit. Above the nebula is the star-cluster 362 H. [Herschel] The star below the nebula is involved in a strong nebulosity. And in searching over this region we meet with delicate double, triple, and multiple stars, which made the survey interesting with almost any power that may be applied.

“For a change we will now try our telescope on a nebula, selecting the great nebula in the Sword. The place of this object is indicated in Plate 2. There can be no diﬀiculty in finding it since it is clearly visible to the naked eye on a moonless night… [being] the only sort of night on which an observer would care to look at nebulae A low power should be employed. The nebula is shown in Plate 3 as I have seen it with a 3-inch aperture. We see nothing of those complex streams of light which are portrayed in the drawings of Herschel, Bond, and Lassell, but enough to excite our interest and wonder. What is this marvellous light-cloud? One could almost imagine that there was a strange prophetic meaning in the words which have been translated “ Canst thou loose the bands of Orion?” Telescope after telescope had been turned on this wonderful object with the hope of resolving its light into stars. But it proved intractable to Herschel’s great reflector, to Lassell’s 2-feet reflector, to Lord Rosse’s 3-feet reflector, and even partially to the great 6-feet reflector. Then we hear of its supposed resolution into stars, Lord Rosse himself writing to Professor Nichol, in 1846; “I may safely say there can be little, if any, doubt as to the resolvability of the nebula; all about the trapezium is a mass of stars, the rest of the nebula also abounding with stars, and exhibiting the characteristics of resolvability strongly marked.” It was decided, therefore, that assuredly the great nebula is a congeries of stars, and not a mass of nebulous matter as had been surmised by Sir W. Herschel. And therefore astronomers were not a little surprised when it was proved by Mr. Huggins’ spectrum-analysis that the nebula consists of gaseous matter. How widely extended this gaseous universe may be we cannot say. The general opinion is that the nebulae are removed far beyond the fixed stars. If this were so, the dimensions of the Orion nebula would be indeed enormous,

Mary Proctor, daughter of Richard Proctor, added her own apt description;

The Orion Nebula Today Now at the beginning of the twenty-first century, the Orion Nebula or NGC 1975-1980 is visually one of the brightest of the nebulae seen in the sky. This magnificent object is a bright emission nebula that is still spectacular even in light-polluted skies. The most modest telescopes can make out the general shape, and its greenish colour. Using even the largest of telescopes, we can observe something new that we have not noticed before. Commonly we call it the Great Nebula in Orion M42 is found lying in the middle of the sword of Orion, or the handle of the saucepan, as seen from the southern hemisphere. In even moderate telescopes it is a truly grand object and words cannot adequately describe this nebula as it truly appears. My own impression of the Orion Nebula is firmly fixed in my head. As the one of the first objects that I observed with a small 60mm refractor, I can still recall being fascinated by the faint, but highly delicate nebulosity. A sharp boundary to darker portions contrasts the ‘bowl’ of the nebula. The most obvious feature is the boundary of light call the Bright Bar. Naturally, when observing it with a much larger telescope, the subtle differences in the surface brightness become obvious, with a mottled surface inter-dispersed with faint filaments. One of the first astronomical photographs taken of this object was allegedly made by American spectroscopist, Henry Draper, at the Harvard College Observatory in 1880. Even today it is a favourite object among those interested in astrophotography or imaging and is truly the most photographed object in the sky! A small telescope, with the short time in guiding, produces truly magnificent results. For the amateur astrophotographer it is likely the first object attempted. It remains a popular photograph in astronomical periodicals, advertising materials and in books.

‘This is an irregular square with a star in each corner, and is situated in the midst of a dark gap in the nebula, within in which the 4 stars gleam brightly… (The Trapezium) The radiant mist surrounding them has a greenish tinge, revealing the vast stellar cloud known as the ‘Nebula in Orion’ with its Isles of light and silvery streams, and gloomy gulfs of mystic shades.’ E. J. Hartung in his 1968 “Astronomical Objects for Southern Telescopes” describes the Orion Nebula as thus;

“One of the most attractive objects in the sky, this great nebula is too well known to need description; …” These possible changes in the whole area are certainly curious. It is hard to reconcile how the observers before Pieresc could have missed such an obvious object, unless there was a sudden brightening of M42/M43 at the time of the introduction of telescopic observations. It is possible, if this hypothesis is correct, that the nebula started rapidly to brighten a few years before the observations of Huygens. An explanation of the mechanism could be the simultaneous ‘throwing off’ of the embryonic nebulosity of the stars in the Orion Nebula. This would cause the stars to brighten suddenly, like FU Orionis. They would then expose the surrounding nebulosity of the whole region to the intense UV radiations of the stars. Over perhaps a few decades or centuries, the material contained in the nebula begins to shine by its own light, becoming the brightest of all emission nebulae.

Illumination of the Orion Nebulosity Many parts of M42 we see in the bright nebulosity are illuminated by stars associated whose structure making the individual parts of the nebulosity appear so luminescent. In reality, this nebula brightness is because of the molecules and atoms contained within the nebula itself. Excitation of this ionised material is by the energies from the many nearby hot stars that are radiating strong UV radiation — causing the familiar fluorescent glow. A large proportion of M42 is possibly illuminated by the star known as Becklin-Neugebauer Object (BNO) or sometimes just Becklin’s Object, which remains visually unseen due to significant absorption of light by the bulk of the nebulosity. Discovered by Eric Becklin and Gerry Neugebauer in 1967, this hidden object appears as the brightest star in the sky at near infrared (NIR) wavelengths (or thermal infrared) at about 2μ to 10μ (microns). Such wavelengths are equivalent to temperatures emitted by the human body. The nature of this star is thought to be a high mass protostar that is in the process of emerging from its embryonic nebulosity from which it recently formed. In more recent times (1993), there was found also another very close object known as IRc2 which seems likely to be a trapezium-like multiple star. Both these stars are some 0.5 pc (1.6 ly.) behind the bright bowl of the Orion Nebula. Origin of the BHO protostar is uncertain and there are several options being considered by astronomers.

!Image,courtesy,of,Marc,Schafer,of,Grahamstown,,South,Africa.!

Visual!Appearance!of!the!Orion!Nebula!as!seen!in!20cm!telescope.!The!pale!green! colour!is!approximately!the!colour!seen!in!the!telescope.!Older!observers!might! not!seen!this!colouration!at!all!due!to!the!colour!loss!under!darkened!conditions,! which!is!mainly!due!to!the!ageing!process.! !!This!image!clearly!shows!the!curve!along!the!edge!of!the!bowl,!with!the! Trapezium!near!the!centre!of!the!frame!—!pointed!to!by!the!small!protuberance! of!dark!nebulosity!from!the!curve!of!the!bowl.!Deep!colour!images!reveal!this! region!around!the!Trapezium!has!carved!out!by!the!hot!energetic!radiation!from! the!new!blue!stars,!producing!the!distinct!hollow!bowl.!

Size of the Orion Nebula In true size of the nebula covers about 1¼ square degrees of the sky, roughly equivalent to be between seven and eight times the apparent area of the Moon. M42 is 66!×60! in size, while to the north is M43 that measures some 20!× 14!. Combined magnitude of the nebula for M42 is equal to any 2nd magnitude star, while the star known as θ1 Orionis has the combined visual magnitude of 2.9v. This star is also known as the multiple star HJ 4581 / h.4581, which was first catalogued by John Herschel. Astronomers have estimated that the nebula’s total masses, is somewhere between 3,000 M☉ and 5,000 M☉ . The distance to the nebula has been presently estimated to be between 430 ±60 pc (370 pc to 490 pc) or roughly 1,400 ±200 ly. Most of the literature now often tends towards 1340 ly. In true size, the bright ‘fan-shaped’ section of M42 is estimated to be about 8 to 15 ly across, while the maximum size revealed by the photographic process, measures at about 30 to 35 ly. We have measured the nebula to approach the Earth at 25 km s-1 (8 km s-1 less than stated by Hartung). The Trapezium is travelling at a slower velocity of −11 km s-1. Using the distance to the Orion Nebula as 410 parsecs or 1340 ±76 light years, means that the trigonometric parallax is 2.571 mas (milliarcsec). If the principal stars in the Trapezium are at this distance, based on the relative position of STF 748A at 6.55V magnitude, then the nature of the system is listed in Tables 1 & 2 (Below.) No one knows for sure about the future of the nebula of M42, but it seems the stellar creation process is still going on. Some have suggested that the nebulosity will become even more brilliant perhaps sometime within the next million years or so.

The!first!photographic!image!to!successfully!reveal!the!structural!features!of!the! Orion!Nebula!was!this!glass4plate!image!taken!26th!February!1883!by!Andrew! Ainsle!Common!(184141903),!an!acclaimed!early!pioneer!of!astrophotography.!This! image!was!made!using!Common's!364inch!reflecting!telescope!at!his!home!in! Ealing,!West!London!in!England.!This!is!the!first!high4resolution!photograph!of!the! nebula!and!clearly!shows!details!invisible!to!the!unaided!eye.!

The man who discovered the Orion Nebula

Leaf!from!the!notebook!of!Nicolas4Claude!Fabri!de!Piersec!describing!observations!he!made!on!or!around! November!10,!1610.!Only!the!left!portion!of!the!page!shows!his!sketch!of!the!Orion!Nebula.!The!right!portion! diagrams!the!passage!of!a!planet!near!a!star,!though!he!text!identifies!neither!by!name.!The!boxes!labeled! “Luna”!and!“Martis”!mean!“Monday”!and!“Tuesday”.!The!telescope!he!used!for!these!observations!has!a! magnification!of!27!or!28!an!an!apparent!field!of!only!about!15!arcmin!or!half!the!diameter!of!the!full!moon.

In his own time, Nicolas-Claude Fabri de Peiresc was known as a savant — French for “a knowing person”, especially a distinguished scientist. Today he would be known as a polymth. He is known to us as the discoverer of the Orion Nebula. a diffuse nebula situated south of Orion’s Belt in the constellation of Orion, in 1610. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. De Peiresc’s interests as a scholar were widely dispersed. He collected ancient gems and coins, he promoted young artists by giving them commissions. Among other things, he ordered from them copies and casts of antiquities, for which he demanded careful accuracy, so that later one or the other copy could be traded as an original. Of equal importance was his interest in the natural sciences. In 1916 Guillaume Bigourdan attributed him the discovery of the Orion Nebula in November 1610. He was also interested in deciphering the hieroglyphs. Peiresc came to travel around Europe with his patron Guillaume du Vair, a French author and lawyer. In 1610, du Vair bought a refracting telescope which Peiresc was able to use to observe the skies, especially the Jupiter moons. Probably in the same year, Peiresc discovered the Orion Nebula. He served in many capacities — advocate, counsellor, judge, professor, benefactor to many promising young scholars. He had also been awarded an income-yielding benefice, the Benedictine

abbey of Guîtres, near Bordeaux. That income would help to finance his non-stop collecting of books, manuscripts, and antiquities of all kinds, as well as his very active correspondence. Peiredc published almost nothing of his own work. Everything we know was gleaned fom his handwritten notes — sometime a tedious job, as the drawing on the previous page attests. His huge archive of carefully filed notes contained the materials for many potential works of scholarship. In every field of learning to which he gave his attention — i.e. almost every field studied in that period. He corresponded with leading practitioners, applying to every topic his sharp critical intelligence, prodigious memory, and remarkable openmindedness.

Nicolas-Claude Fabri de Peiresc, Black chalk drawing by Claude Mellan c. 1636–37. 20 by 14.8cm. Source: The State Hermitage Museum, St Petersburg.

Pieresc was a man of his time, and what a time it was. On a tour of Italy in 1599–1601 he met a wide range of scholars, including Galileo. This was well before Galileo had begun looking at the sky with his new “far-seer”. Pieresc’s discovery of Otion was his own doing. After observing the nebula for the first time, de Peiresc referred to it as a “cloudy nebulosity”. He was not the only one to notice the nebula in the night sky. It is assumed that the first published work regarding the nebula appeared in 1619 and was written by Johann Baptist Cysat of Lucerne in his 1619 monograph on the comets (describing observations of the nebula that may date back to 1611). Also, the nebula was independently discovered by Christiaan Huygens in 1656, who published his findings in 1659. Charles Messier later categorised the Orion Nebula as M42 in his catalog of deep sky objects.

The Horsehead flings its mane Doug Bullis

What we see.

What we donâ&#x20AC;&#x2122;t.

Wait, wait . . . there’s more Energy recorded in the blue-to-red optical band of our eyes is roughly 1% of the full electromagnetic spectrum. We eyepiece-huggers and our grand and glorious gear (mine being among them) can call an object in the sky many things, but a fact in the sky isn’t one of them. We simply cannot see the full set of facts with the eyes and telescopes we have.

It’s hard to beat astronomy when it comes to contrarian canons. It seems that no sooner is a set of facts thoroughly established, than a newer set of facts throws them out the window. Theorems clash with measurements and go off in a sulk — William Herschel was convinced to the end that stars evolved toward nebulae and not the other way around. Once-certain evidence is downgraded from parameter to fond memory. Optical images that suggest stars and galaxies are prima facie evidence that mass shines are shown in other wavebands to be the progeny of energy that doesn’t shine (negative specific heat and angular momentum, for example). In the 1860s physicists were forced to devise an invisible element they called Nebulium to explain the many glowing clouds in the sky that couldn’t possibly glow by themselves. The physicists could not otherwise explain certain lines in the spectra of nebulae that couldn’t be traced to a traditional source. This was, coincidentally, about the same time that James Clerk Maxwell wrote, “All fallow land is to be ploughed up and a regular system of rotation followed.” His off-topic agrarian truism was as important to the future course of science as his formulation of his laws of electromagnetism across the following decade. One needn’t look to the far-off nebulae to see the impossible to be shown as not just feasible but necessary. Any glow-in-the-dark wristwatch will do. The dial’s phosphorescent glow fades across several minutes to several hours. In the 1860s the state of the science was convinced that light should stop emitting immediately when the exciting source goes off. Physicists had to invent Nebulium because nothing else could explain the long-lasting glows of nebulae far brighter than the stars within them — M27 the Dumbbell for example. Barely a decade later James Clerk Maxwell had mathematically shown the opposite. A phosphor is an energised electron that can occupy an energy state whose duration is determined by the pace of energy transfer — today we explain it as old-fashioned angular momentum in a quantised form called atomic spin. The bones of canon litter the landscape of fact. Survey the myriad examples of good laws gone awry a bit more closely and they trace to a root cause:

Contrarians we can see To our eyes, the Horsehead Nebula is a huge daub of shadow upon shadow. In infrared it turns into a patchy glow. Absorption in the visual band V is caused by the same thing that causes emission in IR — gas/dust density. To get a rough idea of how gas and dust interrelate, one dust particle per 1000 gas particles is a good rule of thumb — though thumb sizes vary hugely in this garden. Since astronomer can’t go out there and collect a litre of the stuff, they gauge gas density via adding up the density of all the gas that lies between us and the cloud, then deducting the known amounts of local dust extinction in our locality. Nearly all of the gas/dust in a cylinder with a diameter of the telescope dish stretching to the cloud lies in the cloud itself. If we were to look through an eyepiece, the cloud would look like a flat sheet of a particular light density stretching across the field of view. Fifty shades of gray has a more prosaic connotation here. The measurement of that density is called gas surface density. It is a term that shows up constantly in molecular cloud studies. Gas surface density measurements are commonly made using the 160, 250, 350, and 500 !m bands in the infrared. Since a micron is 1/1000th of a millimetre, this is what the term “submillimetre” in spectral bands refers to. The reports in the literature look a bit arcane, e.g. “Data cubes of 13CO 1→0 (110.2 GHz) and 12CO 3→2 (345.8 GHz) were taken from the Galactic Ring Survey archive (GRS)”. We will elaborate a bit on the technical jargon of molecular cloud studies in the discussion below, but if you can’t live another moment without knowing what all this means, this lecture is a fairly digestible technical analysis of the exact same region we are examining in this report.

Fig. 1 (on previous page): Gas temperature map of the Horsehead, NGC 2023, NGC 2024, and IC434-435 region of L1630 molecular cloud complex. The σ Orinis star that excites the IC 434 H II region is located 0.5° east of the Horsehead nebula.The colours are 12CO (blue), 13CO (green), and C18O (red). From Fig.1 Pety et al 2017: The anatomy of the Orion B giant molecular cloud: A local template for studies of nearby galaxies. Astronomy & Astrophysics Vol. 599, March 2017.

Lynds 1630: The Orion B molecular cloud complex

Gas/dust pockets of varying density give it a bubbling-asphalt look at the ablation surface. A complex webwork or corrugations lie between IC 434 and the Earth, seen in silhouette against the bright nebulosity. (“Bright” is an astringent irony in the eyepieces of visual observers.) In consequence B33 looks like it juts directly towards the quintuplet σ Ori system. Reality is in fact just the opposite: σ Ori is eroding the Horsehead from the top down. Fig. 8 below shows the dust-density structures inside the Horsehead. Although intermingled, B33’s dust clouds are distinct from its gas structures. The latter are slim “Pillars of Creation” type columnar star-forming filaments presently making low-mass protostars. The IC 434 compression front is advancing into the dense (n = 10–100

Lynds 1630 is the southern part of the Orion B giant molecular cloud complex. The 1962 Lynds Catalog was compiled by Beverly Lynds, drawing on Palomar 48-inch Schmidt plates. Most of the 1802 entries are data tables, but pages 184A through F of the 1965 edition of her catalog charts of both bright and dark nebulae on a sky projection. L1630 is a dark gas/dust cloud that is discernible by stellar underdensities in visual band images which emit strongly in IR and sub-mm bands. As shown in Fig. 1 above, the eastern wall of L1630 becomes IC 434, a large HII region being simultaneously compressed and ablated by ionising UV radiation from σ Ori. IC 434 appears edge-on in the visual but sub-mm images reveal it to be an particles cm-3) Orion B cloud at ~7 km uneven compression/ablation surface s-1, triggering the formation of lowtilted downward and to the west (left) mass stars in the densest pockets. The at about 8°. Hα and near-IR images compression front is tri-layered and reveal a vivid array of threadlike parsed in Figs. 12 and 14 below. The streamers emitted from the IC 434 shock front pushing IC 434 into the Ori surface and speeding away from the B cloud has already triggered a minor Fig. 2: The world we are about to explore. surface in the opposite direction from the wave of star formation in pockets near the frontal surface. impacting radiation. (Hence the “Horsehead’s mane”.) The streamers are an The gas surface density threshold for star formation in L1630 is about 129 effect of σ Ori photoionisation along the outer surface of the Orion B gas/dust M⦿ pc−2. wall. A UV ionisation front is a complex 3-layer dissociation field commonly __________ found when O and B stars strongly ionise a molecular cloud surface. IC 434 is For an extended discussion see Spezzi et al 2015: The VISTA Orion mini-survey: Star in effect being shoved en masse into the Ori B cloud by UV and is shoving back formation in the Lynds 1630 North cloud.

The Orion B gas density field There are a number of noteworthy clusters in lower Orion B. The richest, with over 300 stars still embedded in their natal gas, lies behind the halloweenish dark streak that gouges across the NGC 2024 Flame Nebula. This cluster is one of those anti-canonical objects mentioned at the beginning. We take it for granted that the brightest and heaviest stars in a cluster are the first to form and that they stay put in the centre while the lower-mass, fainter stars dwindle in mass and brightness the further out we go. The NGC 2024 cluster is decidedly contrarian: its oldest and brightest stars are spread across the cluster and are about 1.5 million years old. The low-mass infant stars speckle the central region. These are stars only about 200,000 years old. On the other end of the cluster scale, the poorest Orion B cluster comprises 21 embedded X-ray emitting sources inside NGC 2023. In our eyepieces, NGC 2023 is a bright reflection nebula about 400 pc (1304 ly) from Earth that is so unassertive that we regard it more as a signpost pointing to the Horsehead than a target in its own right. Its modest display belies a complex heart. The whitish glow we see is actually reflected light from nearby luminous stars. Inside, NGC 2023 is a heavily dust-obscured seething nursery of four star-forming cores. The brightest is illuminated by the B1.5 MV 7.8 star HD 37903. There is a blister pocket in NGC 2023 (pictured in the discussion below) that somewhat resembles the θ Orionis (Trapezium) blister we see as the billowy beauty called the Orion Nebula. Several mm-band spectral signatures in NGC 2023 cannot be identified with optical or near-IR counterparts. This suggests the presence of a more deeply embedded population of protostars hidden within the densest molecular cloud cores surrounding NGC 2023. __________ In case you are seized with an urgent desire to become a gas/dust specialist, here are a few brush-up papers: 1, 2, 3. If you remain so inspired and your study habits match your ardour, take our word for it, you will be welcomed with open arms and a steaming cup of coffee at any radio observatory.

Fig. 3: The Galactic magnetic ß-field is markedly warped and twisted throughout the OrionMonoceros molecular complex. Since electron transport along magnetic fields carries dust with it, measuring the dust emission is a tracer of electron density. Electron density in turn indicates the strength and direction of the ß field. When the Planck mission data-delivery component was first being designed, the parameters included a sensor for the 353 GHz emission band, which arises from the interaction of polarised dust particles with the local magnetic field. The dust tends to align its axis with the field’s poloidal axis (the N - S line). Hence dust alignment is a probe of magnetic field alignment. The density of the dust in turn traces the local temperature. The above cutout from the Planck whole-sky map is actually a dust density/ temperature map which also tells us what we want to know about the Galactic magnetic field in this region. Source: ESA Planck 353 Ghz polarised emission from Galactic dust.

STAR CLUSTERS 101 We need to step back a bit to understand why star formation in photocompressed features like IC 434 differs from the other three main types of star cluster formation, free-fall collapse, triggered, and cloud-collision. Entire volumes have been written about each of these because their different physical processes produce different types of star clusters.* A 1000 light year diameter pancake-shaped volume inside a spiral arm (e.g., the Orion A and B molecular complexes) will show past and present evidence of all three. Learn how these processes produce different types of star clusters, and with some practice you will be able to simply look at a cluster and you can make a fair guess how it was formed and how old it is simply by knowing the origins of the star distribution and magnitude patterns that put it there. Free-all collapse The traditional image of star cluster formation in a coffee-table book is free-fall collapse. Here, a large molecular cloud entering a spiral disc from above or below fragments first into large lumpy filaments, and then denser clumps and finally very dense cores. What the coffee-table images donâ&#x20AC;&#x2122;t show you is that a galaxy disc is a hostile, violent place filled with divisive supersonic shocks and meandering magnetic fields. Turbulent shocks advance at many times local sound speed (rule of thumb: M 1 = 200

Fig. 4: In triggered star formation, compact high-mass gas clouds collapse into clusters of high-mass O and B stars which radiate enormous amounts of UV radiation. This initiates a rapid gasclearing shock wave. Several million years later, supernovae compress the ring into arcs of clusters.

m s-1 in a 10 K gas parcel of 10â&#x20AC;&#x201C;100 atoms cm-3). Turbulence fragments a gas cloud, magnetic fields tend to redirect its ions. A given clump of gas/dust of n = 104 atoms cm-3 cannot free-fall into star-making densities of 10,000 to 1 million atoms cm-3 until the ambient velocity field around the cloud goes subsonic. Turbulence and magnetic fields must wear each other down until free-fall densities and subsonic surroundings match, a point at which you can start believing the coffee-table depictions again. Triggered star formation typically occurs along the rims of expanding bubbles of gas ejected from the cores of very hot, O-star dominated young clusters. The ejected gas occurs in two stages: first, gas clearance or stellar

feedback by radiation pressure (as we see happening in the Rosette Nebula right now) is followed by supernova shock waves as the cluster's most massive stars detonate. The shock waves sweep into the previously expelled gas, driving it into the surrounding gas of the galaxy. Smaller local clusters then collect and collapse out of the expanding shells (see drawing above). Cloud collision occurs when two massive molecular clouds smash into each other at high speed (>10 km s-1). Star clusters form along the compression zone. M20 the Trifid is a low-mass example. The supermassive young clusters Westerlund 1 & 2 and NGC 3603 were high-mass collisions.

__________ * Their popularity is enhanced no end by the fact that thousands of Ph.D theses remain to be mined from so complex and mathematically intricate a phenomenon.

Out of the murk: How astronomers turn dark into bright

Fig. 5: After all the radio receiver numbers are crunched, a typical gas surface density plot looks like this. The data compressed into each square is called a data cube. When all the data cubes are plotted into a flat sheet, a computational algorithm translates the individual wave band densities (squiggles above) into contour lines, which then reveal the actual shape and density of the clouds. The one decisive component missing in this type of measure is the local temperature inside each data cube. Temperature is vital because the atomic reactions that make molecular hydrogen out of atomic hydrogen take place mainly on the surfaces of brutally cold (<10 K) dust particles. The most important takeaway from these two images is how thready and clumpy the realm of dark molecular clouds is when seen in perspective. The Orion B cloud shares these same properties, which is why we take such delight in the contents of such a small region. From afar it looks like Fig. 2. Close in it is filled with such visually dissimilar objects as the Horsehead, IC 434, the Flame Nebula IC 2024, the star-making cloud IC 2023, and Sigma Orionis.

Fig. 6: Even dark clouds behind bright clouds can be measured using infrared. Infrared dark clouds (IRDCs) are a hot topic in galactic astronomy these days because they are future star-forming regions that we would otherwise have no other way of knowing. Here is one of the most massive IRDCs nearby, named G28.37+ 0.07 because of its position very close to the Milky way equator and in the densest part of the Galactic disc. If we were at the same distance to G28.37+0.07 as we are to the Horsehead, it would be much larger and very much darker.

THE HORSEHEAD NEBULA For all its vaunted status as a prize catch among amateur astronomers, the Horsehead is an even better catch to devotees of star formation. It is a halfmillion year old gas/dust cloud with a virial mass of ~56 M⦿, divided about equally between the two clumps shown in Fig. 6 on the next page and the less-dense gas in the “head” and “neck”. The upper gas/dust core is called SMM1 and has a mass of ~22 M⦿; the lower core SMM2 is 34 M⦿. The two cores are gravitationally self-bound. This is important for our understanding of the way star formation occurs in “pillars of creation” types of structures in which a dense column is being eroded by UV radiation on one end while at the same time the column is forming stars in dense cores in the middle. The gas column density of the SMM1 core is densest on its east (top) side while its temperature is the warmest on its west or bottom side. These facts suggest that both the column density and velocity width of the core are greatest at the core centre. This in turn suggests that infalling motion into the centre is about 3.0 km s−1, which is well above the free-fall value. It seems counterintuitive given the HH’s appearance that it is not a static structure. It is losing mass thanks to σ Ori’s UV. Surprisingly, the Horsehead also has a rotational moment. Its emission in radio bands are Doppler shifted as different parts of the nebula move toward or away from us. The horse’s nose is turning toward us and part of the mane is turning away. The Horsehead’s rotation has deformed an otherwise unremarkable gas/dust pillar into a distinctive trope that attracts us like no other shape in the sky. Shorn of the visual poetry, the cloud’s rotation introduced a centrifugal component into a gas mass that torques it normal to the axis of rotation. The horse’s nose and mane turn anticlockwise from the neck, thereby giving the nebula its distinctive liveliness. Without rotation, we mightn’t be seeing a horse.

__________ * The term “virial” is important to an understanding of gas dynamics and star cluster evolution. The underlying principle is easy: An object’s potential energy is twice its kinetic energy, under the sensible idea that if it takes a certain amount of energy to move an object from point A to point B it takes the same amount of energy to move it back. P = 2K. Works great, till the mathematicians get hold of it.

Fig. 7: The Horsehead and IC 2023 from the VISTA (Visible and Infrared Survey Telescope for Astronomy) J, H and Ks filter set in the near-IR, processed by Robert Gendler. The distance from the tip of the nose to the back of the neck is ~4 ly, about the same as between the Sun and Alpha Centauri. Note the prominent blister formation in NGC 2023 caused by photon pressure from its central O star SAO 132464.

Star forming cores in the Horsehead

What makes a molecular cloud “molecular”?

The renowned “Pillars of Creation” in M16 were long ago discovered to be forming stars inside those dense clumps. Over the years many other photoablated columnar dark clouds have been to found as actively forming stars inside even as they are being withered away from the outside. The Horsehead is no exception, Core A in the Horsehead Nebula (the top of its head, SMM1) has a mass of ±22 M⦿. The column density of this core is densest on its east side (brow), while the temperature is highest on its west side (the crown of the head) at 22 K . These observations suggest that ionisation of the Horsehead’s molecular gas by IC 434 behind it proceeds from the west side of the core. That in turn suggests that the Horsehead is not the vertical column we perceive it to be, but that its head is slanted away from us. Core B (the “neck” overdensity, SMM2) lies not in the columnar centre but in the boundary region between the Horsehead and NGC 2023, i.e., nearer to us. That, too, would be more likely if the column was slanted with the bottom end nearer to us than the top. In both clumps the column density and velocity width of the core increase toward the core centres. This is a signature of infall, which means the ionising disturbances affecting the Horsehead’s exterior do not much change events going on inside. The velocity of the infalling is estimated to be ~3.0 km s−1, consistent with the free-fall velocity of a gas mass of 25 to 30 M⦿. In addition to the gravitationally bound state of both cores (suggesting ongoing star formation) the temperature of the cores also increases toward their centres. All these are signatures of gas infall leading to star formation, which in turn suggests that the ionising disturbances affecting the Horsehead’s exterior do not much change events going on inside. Despite the fragile look in Fig. 5, there’s a lot of life left in those flaring nostrils. The Horsehead is a very complex place chemically, especially in its organic molecules. (“Organic” in astronomy only means that the molecule contains

Molecular clouds (MCs) are dense pockets of self-bound gas whose masses range between 104 M⦿ to 6 x 106 M⦿ and whose sizes range from 10 to 100 pc (32.6 to 326 ly). There are about 4000 known MCs in the Milky Way. MCs are inhomogeneous, comprising multiple thready filaments of varying shapes and sizes and a number of higher-density regions termed clumps and cores. While they are called “molecular”, most of them are composed of approx. equal parts of atomic H1 and molecular H2 gas, plus varying amounts of dust. The molecular gas condenses in a core, while the atomic gas forms a thick shell around it. The shell absorbs starlight, which allows the core to become extremely cold, ~10 K. While they may be large dimensionally they are also very airy, with average densities of only about 100 atoms per cubic centimetre. As a rule of thumb, the densities of clumps range from 100 to 1000 atoms cm-3, while cores range upward of 1,000 to a million or more atoms cm-3. While local conditions vary dramatically, in general the density required for a molecular cloud to begin inward free-fall into star-forming densities occurs when a given core reaches 10,000 atoms cm-3 and the two divisive forces of supersonic turbulence and magnetic field pressure have weakened each other so neither can impede free fall.

carbon.) Just a few of these are H2CO, CH3OH, CH3CN, CH2CO, CH3CHO. The production of molecular species like these occurs via grain surface reactions in a UV-dominated photo-dissociation region.

Fig. 8: The Horsehead in 850 μm IR showing emission from two prominent dust clouds labeled SMM1 and SMM2. Photoionisation from σ Ori at the surface of SMM1 projected a shock front into the cloud and compressed it, forming the two clumps shown here. These will eventually become gravitationally unstable and collapse into new stars. Source: Meyer et al 2008.

Alnitak

spectroscopic pair. σ Ori B, the least massive member, is B2. Their total

The easternmost star in Orion’s belt is a triple star (A-B-C) system at the end of Orion's belt. The triplet is energetically associated with both IC 434 and IC 2024 the Flame Nebula. Alnitak A is a mag 2.0 type O9.5 blue supergiant of 33

luminosity L≈ 7.6 × 104 L⦿, most of which is in the FUV. What besides these 5 stars makes up the σ Ori cluster? The initial mass function (IMF) of star clusters divides into two classes: stars heavier or lighter than 0.5 M⦿. To trim the frills a bit, most gas masses large enough to form multiple supermassive stars also form a great many brown dwarfs, red dwarfs, and sub-solar mass stars. Because of the bright stars’ glare, few of these can be spotted visually (see Fig. 10 next page), and sometimes vex even millimetre

±10 M⦿, Teff 29,100 K and 250,000 L⦿. It is 6.4 Myr old, ripe middle age for an O9.5 supergiant. Its spectroscopic companion Alnitak B is a 14M⦿ BIIV blue subgiant with a Teff 29,000 K which is 7.2 Myr old. Alnitak’s third companion C is a blue type BIII subgiant, barely within the emission threshold for the far UV radiation that is the most destructive to nearby gas/dust clouds. A star cluster containing Alnitak-mass stars would have cleared its natal gas by about 3–4 Myr after initial formation. Hence the Alnitak triplet’s UV radiation today is fully available to fluoresce nearby dust. Is the Flame Nebula within Alnitak’s fluorescing radius? Hipparcos distances for Alnitak A-B-C triplet are 1260 ± ly, while IC 2024 the Flame Nebula is 900–1500 ly distant. This casts doubt on Alnitak being the Flame’s only flame. This fact inspired a deeper look using the Chandra X-ray and Spitzer IR imagers. The actual Flame’s flame turned out to be an O star obscured by 24 magnitudes of overlying dust. We see 0.000,000,000,255 of what's actually there. You get better odds with a single Lotto ticket than with seeing a photon from the middle of the Flame.

and X-ray band specialists. The σ Ori cluster was first resolved down to the MV 19 T-Tauri X-ray emitters in 1994 when over 100 were pinpointed within the σ Ori virial radius. In 1994–1995 Frederick Walter and 3 other astronomers successfully traced 104 σ Ori pre-main sequence infants out of some 300 candidates. Their discovery image is on the next page. None of the image’s circled stars are visible above mag 16 in the optical, but a handful of brighter cluster members were conveniently plotted on a sky image on Wiki.

σ Orionis σ Ori is a bound quintuplet containing an O9.5, B0.5, two B2, and an A2 star with a FUV luminosity of 2 × 1049 photons each second and a total luminosity of 76,000 suns. It is about 385 pc (1250 ly) from us while the Horsehead is a little closer at 350 pc (1140 ly). σ Ori AB are responsible for energising the seemingly far-off IC 434 emission nebula. The pair is a 3 Myr old close spectral type O9.5V and B0.5V binary. The closest pair AB are only separated by 0.2”– 0.3". In 2011 σ Ori A was shown to be a

Fig. 9: Gentlemen, when it comes to star cluster formation in giant molecular clouds, it’s turtles all the way down.

Clusters of clusters Clusters form in clusters. The filament–clump–core hierarchy of gas masses in giant molecular clouds implies that as the cloud enters into the Galactic disc, the multiple shock fields in the disc will interact with the infalling gas and initiate a multi-million year sequence of star cluster formation. The overall process can be described as a 3-way tug-of-war between the disc’s supersonic turbulence bubbles, magnetic fields, and the cloud’s own self-gravity. A specific gas cloud’s gravity needs only to hold up against the pressures until turbulence and magnetic fields wear each other down. The cloud can then infall into a star cluster whose mass and number of stars depends on the cloud’s initial mass (IMF = initial mass function). Extrapolate this over dozens of clumps and you have the 20- to 30-million year cycle of star cluster formation of the Orion complex.

Fig. 10: The σ Orionis star cluster, taken from the 1997 discovery paper. The mag 3.8 σ Ori triplet is vastly overexposed here to bring out the faint, low-mass members. Visually we see 7 stars, though two are non-cluster stars. The circled stars above are members of the cluster. The cluster is very young at c. 3.6 Myr old. Along with the few higher-mass and brighter stars shown in Fig. 9 to the right, there are 104 pre-main sequence stars in the cluster, a sign of its youth. Young it may be, but the σ Ori AB UV-emitting duo has still managed to ionise the entire IC 434 nebula and driven a large quantity of its gas deep into the Orion B cloud.

Fig. 11: This image map from Wiki shows the σ Ori cluster stars visible in typical amateur telescopes of 6” and 8” aperture. The original Wiki image identifies the stars more clearly. This image also reveals the fine structure of the magnetic field lines jetting away from IC 434’s photo-ablation surface as champagne flow. (See p.16 below.) Next time you visit the dentist How did the “X” in X-ray get there? When an unidentified form of electromagnetic radiation that could penetrate human soft tissue but not bone was discovered by the German physicist Wilhelm Röntgen in 1895, the nature of the rays was a mystery. He named them “Strahl" rays. In German the orthographic form for the letter X is “Strahl" (in English it is “ɛks”).Translators into English and other language dispensed with the formalities and used the letter instead, Strahl rays became X-rays. 12 101

IC 434 Sigma Orionis (σ Ori) AB’s massive UV wind has been pushing against the nearly perpendicular surface of IC 434 for several million years. During that time the wind has gradually accelerated the entire IC 434 wall toward the west and into L 1630, the whole huge thing including the z-axis surface fore and aft across which we gaze. Its present velocity is 3.5 km s-1 into L 1630. If we were astride the Horsehead, σ Ori would be a stellar dot about as bright as the gibbous moon. If we were on σ Ori looking back the Horsehead would be a round blob larger than the Coal Sack and very much darker, while the front surface of IC 434 would look a bit like a faint red Aurora Borealis covering nearly half of the sky. IC 434 is squeezing into L 1630, the Orion B cloud, shrinking out features like NGC 2024 the Flame Nebula, the star-forming reflection nebula NGC 2023, the Horsehead, and all the other structures we see to the west (left side) of IC 434. One seemingly feckless star in our eyepieces is responsible for the whole shebang: σ Ori. The velocity of the wind that leaves σ Ori AB is 35 km s−1. The σ Ori AB system itself is moving toward IC 434 at 15 km s-1 — creating a rare dust bow wave instead of a more common gas shock front (pointed out in Fig. 1 above). Hence its particle wind arrives at IC 434 ± 50 km s-1 as a supersonic shock moving at Mach 5 with respect to the local sound speed cs. While σ Ori’s photons arrive at IC 434 in ~4 years, its ionised gas takes roughly 150,000 years to get there. Even given dramatic difference in time scales, today σ Ori’s never-ending onslaught of UV radiation and supersonic particles arrives at IC 434’s surface as a two-component fluid of dust and gas particles acting as a single energy field. Each component — photons and particle wind — has its own density, temperature, velocity, mass, and response to magnetic fields, but their concomitant effect is a nonstop wavefront wall forcing its way into IC 434 at Mach 5. When the field arrives at IC 434’s surface, three sequential interactions occur: photo-ionisation, photo-irridation, and photo-dissociation (commonly acronymed

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Fig. 12: This is a somewhat oversimplified, depiction of what happens when ultraviolet radiation from a hot O or B star arrives at the surface of a cold molecular cloud. The incident stellar UV radiation from σ Ori arrives from the left at the cloud surface as a vast front, much the same as starlight enters a telescope as a flat wavefront before it is refracted or reflected. In a molecular cloud the ionisation or photo-ablation front forms the outermost surface of the cloud. Incoming FUV and EUV photons interact with atomic hydrogen and oxygen there, ionising the hydrogen to ~10,000 K. FUV photons that pass through the surface penetrate into a 1500 AU (0.03 pc) buffer zone of atomic hydrogen mixed with ionised carbon, atomic oxygen, and dust at between 500 and 1000 K. Dust surfaces are formation sites for H2 and polycyclic hydrocarbons (PAHs) when the dust is a cold 10 K, but when heated to 500 K up to 1000 K in the buffer zone, the dust particles lose into their surface layers of atoms and ions but remain intact as dust particles. At the base of the atomic hydrogen layer lies the dissociation front, where photons with enough energy to break the 4.75 eV binding energy of H2 into two H atoms. By now, so much incoming energy has been absorbed that the cloud cools to its core molecular temperature ~10 K. IC 434 can be likened to a celestial game of Pacman in which the munching monster chomps its way through the whole cloud. Source: Goicoechea et al 2016.

PI, H1, and PD). Each one is responsible for one of the end-game phenomenon that we marvel over when seen in an eyepiece: an emission gas/dust cloud, a dust clump, an emission gas nebula, and a reflection nebula. This eye candy is high-energy but low-cal. It is easiest to understand what happens to IC 434 under σ Ori’s photon barrage by likening IC 434’s surface to the three colour emulsion layers in predigital photographic film: cyan, yellow, and, magenta. Incoming light passed through the film’s pigment layers, but the specific emulsion layers absorbed only the light whose energy corresponded with that layer — cyan, yellow, etc. Similarly, if we follow the wavefront of σ Ori UV radiation as it enters and passes through IC 434, the UV passes through three layers of physical reaction. Each has its own effect on the photon wavefront. Fig. 13: The ionisation surface of a molecular cloud is somewhat analogous to the chromosphere of a star. Magnetic fields anchored in the photosphere at one end are carried by the solar wind into space, resulting in open magnetic fields and a channel for the fast solar wind. Source: Prof. K. Lang, Tufts University.

Photo-ablation (PA, aka photo-ionisation PI) Compression and fragmentation of UV-irradiated cloud fronts is a common event in the vicinity of young massive stars. Far-UV (FUV) radiation with wavelengths between 912 Å and 2,000 Å heats the surface layers of a molecular cloud. The radiation’s penetration depth is determined by the amount of light the dust absorbs and re-emits at a lower wavelength. FUV radiation heats the gas/dust surface to ~10,000 K. The topmost interaction layer stretches from just above the surface of IC 434’s atomic hydrogen layer all the way out into deep space. This is the origin of the Horsehead’s mane that we see streaming away from the high-density, high-temperature surface of IC 434 into a low-density, low temp medium of not much, really, at all. The name for IC 434’s ejective energy transport is “champagne flow” because it is a gigantic version of what happens when you pull the cork on a bottle of bubbly. Fizz. Lots of it. Call it what you will, the reality is that it’s just heated gas expanding away from the hydrogen ionisation layer and back towards the UV source (σ Ori). It

Iron filings in space Where there is electron flow there is a magnetic component. Roughly 95% of the particles streaming away from IC 434 are negatively charged electrons. At ±10,000 K the charged particle wind of electrons plus heavier ions of other atoms is too energetic to recombine with protons back into hydrogen. They flow out into space perpendicular to the surface where they were generated — which hurls them back toward σ Orionis. We can’t see the electrons, but we can see their radiation as HII in near IR. The thready “horse’s mane” effect we see is caused by magnetic field lines induced by parallel electron flow. For astrophotographers those parallel filaments make a challenging fine-grained target. The ancestral roots of parallel flow lines trace back to André-Marie Ampère, who noticed that parallel wires with currents attract one another if the currents flow in the same direction. In space the analogous effect is magnetic confinement within flux tubes.

travels at roughly the local sound speed, in this case ~10 km s-1. __________ For further reading see Bally et al 2018: Kinematics of the Horsehead Nebula and IC 434 Ionization Front in CO and C+. Astronomical Journal, Vol 155, No 2. 14

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Photo-dissociation (PD) The next reaction to take place is photo-dissociation. The actual surface, the leading edge, of IC 434 is atomic hydrogen. It is about half to a tenth of a light year thick, with minor local variations (Fig. 14 right). Photodissociation is a chemical reaction in which energetic UV or Xray photons break down chemical compounds. In molecular clouds like Orion B, photo-dissociation mainly affects H2, which is broken into one hydrogen atom and a free proton plus energy. Photodissociation is not limited to gas clouds in space. Any energetic photon can affect the valence bonds of a chemical compound. Closer to home, photosynthesis via chlorophyll in plant tissue and sunburn on unprotected skin are also photodissociation effects. In IC 434, some of the photons that penetrate through the surface of the cloud’s atomic hydrogen “skin” interact with atoms in the 0.8 pc (0.25 ly) atomic hydrogen layer beneath. The H layer sees a temperature drop from 1000 K to between 300 and 500 K and the next layer down, the molecular hydrogen H2 layer. Any photons that reach this far into the cloud have enough energy to dissociate H2 into H1, but they do affect ionised carbon C+ and carbon monoxide, CO. By the time photons have made it through the dissociation layer, their energy has been so depleted that they end up merely compressing the gas surface instead of dissociating it. Further inward toward the cloud core, the temperature drops rapidly below “freezing” at 100 K, till a few pc further inward the cloud is ~20 K and 30 times denser than the atomic hydrogen layer at its surface. Taken together, the PI, H1, and PDR layers somewhat resemble geological strata on Earth. Folding and buckling affects all the strata, not just the individual layers. Out in space, the undulating surfaces of molecular clouds in astro images are caused by varying density and composition in small-scale local structures. Each structure responds in its own terms, but they are all impacted by the relentlessness of an O-B star’s wind.

Fig. 14: This simulated cross-section of the photo-reactive layers across a molecular cloud surface depicts the Orion Bar molecular structure near the Trapezium in M42. There is no comparable image-based sim for IC 434, but the basic cloud orogeny is similar. The incident stellar UV radiation comes from the Trapezium. The temperatures of the advancing ionisation and dissociation fronts are reconstructed from VLA mm and ALMA sub-mm bands. The dusty photoionisation layer and the red molecular photodissociation layer are clearly separated by the atomic hydrogen gap. Source: Goicoechea et al 2017.

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Champagne flow

When a atomic hydrogen shell of a molecular cloud is ionised by UV radiation, the pressure balance between the cloud and its surrounding gas is disrupted. Typically, gas at the ionisation surface reaches ~10,000 K. The hot gas exerts enough pressure on the ambient gas to punch through the gas at

supersonic velocity into the surrounding low-density medium. IC 434’s pressure imbalance will continue until σ Ori’s energising UV radiation stops. Since the L 1630 molecular cloud is very large and σ Ori’s lifetime is not, one day, in time out of mind yet to be, the horse’s mane will stop flailing.

Fig. 16: Most people might think that sampling 20 different styles of champagne to better Fig. 15: Champagne flow can be describe as an understand the behavioural differences in their bubbles would be a rather jolly affair. Scientists take unconstrained pressure field of magnetically vectored a more straight-laced approach. The source paper for this image is, to turn a phrase, dry. ballistics. The difference between bow shock Astronomers might do well to learn how terrestrial studies of champagne flow dynamics provide a compression and champagne flow release is a study of richer knowledge experience than a study of the gas expansion parameters derived from ALMA contained versus unobstructed flow. Flow fields in data. There is really no other way to arrive at wink-of-the-eye conclusions such as, “empirical space are not constrained by the walls of celestial flute glasses, but they are affected by local gas overdensities relationships reveal the bubble diameter to be proportional to the cube root of the vertical displacement“. Would that ApJ, A-J, A&A, JASP, and MNRAS could be such an entertaining read. and magnetic fields. Source: Cyganowski 2003. 16 17 105

NGC 2024 The Flame Nebula

Innies -vs- outies

At 415 pc (1350 ly) the Flame Nebula is outside the reach of the Alnitak Outside-in star formation sequences are well known in dwarf spheroidal triplet’s fierce far-UV radiation. What then lights up the Flame? There being galaxies. There, gas ejection from the bright first-gen stars loops high above the no other bright stars nearby, the illumination must come from within. The task galaxy but infalls gradually back into the core. Many dwarf galaxies exhibit a of finding the illuminating source is not helped by dual- or triple-tier metallicity ratio that trace this that bleak gash of black lying in front of the history. The majority of dwarf star formation emission nebula. Alnitak’s dazzle doesn’t help. occurred 10–12 billion years ago when the universe’s At the centre of the Flame Nebula Chandra Xgas was much denser but also much hotter. Dwarfs ray data found a cluster of newly formed stars; 86% accreted abstemious quantities of fresh gas, but did it of them evidence circumstellar disks which suggest for billions of years. very young star clusters still in the mass assembly Star clusters are quite different. The standard stage. The cluster’s total estimated population is scenario is that they accrete large amounts of fresh about 800 stars, most of them low mass cluster gas over a few hundred thousand years, then eject members a mere fraction of the mass of the Sun. the excess over the next few million years. Standard cluster IMF functions predict a large There are several explanations for the inverted population of red and brown dwarfs. stellar distribution in NGC 2024. One is that the older An ESO Chandra X-ray and NASA Spitzer Space stars have had more time to drift away from the Telescope survey of NGC 2024 revealed that the stars centre of the cluster, or be kicked outward by 2-body on the outskirts of The Flame’s central cluster are interactions with other stars. But that does not about 1.5 million years older than the stars in the answer the question of why there are two centre, which are roughly ~200,000 years old. generations. Why didn’t all the stars form at once like Chandra data trace the brightness of stars in Xmost other clusters? rays, which is a direct probe of their masses. Another plausible explanation is that star Combining those data with Spitzer and 2MASS formation continues in the inner regions because of data for the stars’ brightness in three IR bands gas, not stellar, migration. Such would happen if the revealed the central stars were faint and young Fig. 17: Gas temperatures in the Orion B molecular gas in the outer regions of a star-forming cloud is while the bright stars in the outskirts were the bright thinner and more diffuse than in the inner regions. complex. The scale on the right is in Kelvin. The and old. Over time, the outer gas density infalls till it reaches crack-like features are centrelines of dense dust A similar outside-in star-forming sequence replicates the critical density to collapse into stars, thus clouds. The IC 434 photo-dissociation zone is in the M42 Orion Nebula cluster. The 1.2 million year old suggested by the pale green arc on the right. Note explaining the outside-in evidence. Star formation Trapezium lies in a cluster whose outlier stars are aged 2 that the Horsehead absorbs radiation along its ceases in the outer region, but there is still enough gas million years. The Flame’s second-gen stars are dim in the interior to form the sparse population of low“neck” but emits millimetre-band radiation at the dwarfs while M42’s are intensely bright supergiants. mass stars in the Flame’s baby crèche in the middle. top of its “head”. We can also see why the name “Flame” Nebula is such an apt name.

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Fig. 18: The dark gash in the Flame Nebula that we see in our eyepieces and images is caused by a massive dust filament which is not associated with the Flame Nebula but simply lies in front of it. It is the optically visible shadow of a much larger, longer filament hat spans the entire lower Orion B cloud complex. Much of the Flame’s optical glow is emitted when free-free electrons and protons fall below the 13.6 eV ionisation threshold and recombine, emitting visual Hα and Hß lines.

Fig. 19: The Flame’s bimodal distribution of a few bright giant stars amid a large number of petite subsolar stars and very few mid-mass stars in between is obvious even in a simple visual examination. The bright star IRS 2b pointed out with the yellow dashed arrow is responsible for illuminating the entire Flame Nebula. It exhibits the IR flux density typical of a late O9/early B0 star. The entire central cluster is obscured a remarkable 24 visual magnitudes by the overlying gas/dust filament shown in Fig. 16 left. Without the overlying dust IRS 2b would be a dazzling bluewhite rival to nearby Alnitak. 18

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Fig. 20 Diffuse gas, filaments, and dense condensations are analogues of muscles, bones, and vital organs in our bodies. Emission from carbon monoxide, water, hydroxyl, methanol, and other organic molecules reveal molecular secrets the way Magnetic Resonance Imaging (MRI) reveals the internal constitution of our bodies. Source: Natalie Ruiz, Molecular Astrophysics Group, 03/07/2017.

Fig. 21: The Chandra probe detected 283 X-ray sources typical of T-Tauri and earlier protostars in the Flame’s central cluster. Such stars would be low-mass (<0.1 – 0.8 M⦿) stars only a few hundred thousand years old. They are distinct from the cluster’s 1.5 million year old giant stars.

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NGC 2023 NGC 2023 is the Orion Nebula’s wallflower cousin. Both are essentially big billowy bubbles, though M42 goes about the matter a bit more gaudily attired. Both are illumined by an overwhelmingly bright star — O supergiants suffer from terminal dazzle. Both have closets full of comely clothes — bright emission veils, ink-dark globules, jewelbox sparklers, a taste for subtle spectral pastels. While M42 tries its best to outshine the neighbourhood, NGC 2023 enjoys the prettier view — who wouldn’t want a towering dark horse flinging its lucent mane as a next-door neighbour? And yet NGC 2023 is resoundingly ignored, even by the professional community: Simbad cites 3557 papers with “M42” in the abstract; NGC 2023 makes do with a modest 553. (OTOH, take a look at those author names as a testimony to astronomy as a beacon of nonparochial thinking.) Step a couple of centuries back and William Herschel’s discovery note on Sweep 352 dated 6 Jan 1785 described it as "A bright star with a very considerably milky chevelure [nebulosity]; a little extended, 4’ or 5' in length, and near 4' broad; it loses itself insensibly. I suspected some extensive milky windings in the neighbourhood but could not verify them; other stars of equal magnitude are perfectly free from this chevelure." Even so indefatigable an observer as America’s Steve Gottlieb gave it short shrift: “13.1-inch telescope: fairly prominent nebulosity surrounds mag 7.8 SAO 132464. The Horsehead nebula lies 15' SW”. Poor thing, upstaged yet again by its vexatious but beguiling neighbour. Let’s redress the imbalance a bit. NGC 2023 is the site of the first detection of fluorescent H2 emission in IR in the mid 1980s (1, 2, 3, 4). Half a decade later the first optical detections of rotational H2 transitions were made there, some 30+ spectral lines from highly excited H2. It is also where Extended Red Emission (ERE, 6000–7400 Å) was discovered. It is one of the reflection

Fig. 22: The NGC 2023 gas clearance expansion shell is often referred to as a blister nebula, but the two terms describe the same type of event. These are champagne flow still in the bottle. All star clusters end their active star formation era with an excess of unused mass. High stellarmass clusters with stars more massive than B3 shed the excess by photo-evaporation. These stars radiate mainly in energetic UV whose radiation pressure ejects the gas at high velocity. Here the ejecta bunches up as it works its way through the natal cloud. When it encounters the low ambient medium outside the star-forming cloud it will burst free into champagne flow. Try this with your pals after your next star party and you’ll agree.

nebulae in which the infrared features at 3.3, 6.2, 7.7 and 11.3 !m were first traced to polycyclic aromatic hydrocarbons (PAH) (1, 2, 3, 4). PAHs may sound arcane and abstract, until you realise that they make up much of the Earth’s coal and oil — and therefore forest fire smoke. Impressive record indeed for a wallflower. NGC 2023 is a bright photoionisation nebula illuminated by the B1.5 star HD 37903. Contrary to its low-key presentation, NGC 2023 is one of the brightest reflection nebulae in the sky. Even so, it is used more often as a guidepost to the Horsehead than an an intrinsically interesting object in it own right. In 3D space it is located just east of the Horsehead nebula but somewhat behind it, in between the Horsehead and the deformation surface that gives us IC 434. It was born 8–10 million years ago out of the same class of high-density compact molecular hydrogen cores that lie quietly in waiting while magnetic fields and turbulence battle for dominance in the surrounding region. There are quite a number of these objects in our galaxy; they commonly go by the name of compact high-velocity clouds (CHVCs). See 1, 2, 3, 4 to understand why they are more important than they seem, NGC 2023 being a case in point. When turbulence and magnetism both lose in their battle and lapse into quiescence, clouds like NGC 2023 free-fall into clusters. NGC 2023’s protocloud was massive: between 100,000 and a million M⦿. The B1.5 star HD37903 has a surface temperature of 22,000 K and is responsible for the excitation of gas and dust within the nebula. It lies near the front side facing and near the edge of its molecular cloud. Off-centred progenitor stars are not uncommon: the Trapezium stars in M42 are even further off-centre. We can readily see why in both cases: one side of the expanding shell is encounters more resistance from dense gas, while the other side is pushing into lower density. No matter where you are, space has its hills and hollows.

In addition to the reflection component, a number of Herbig-Haro objects (HH) which are associated with pre-main-sequence stars exist in the surrounding dust clouds. Two HH objects, HH4 and HH5 in the southeastern part of the nebula are illuminated by a star designated star C. Star C is thought to be a T-Tauri star. The presence of these very early stars suggests that NGC 2023 is still an active star forming cloud. Despite its timid showing of only 21 stars, NGC 2023 is one of the most complex nebulae in the entire Orion A and B Cloud complex. This wallflower is the queen of the garden. Fig. 23: This narrowband image of vibrational fluorescence in N2023 is just one spectral line in the over 30 lines arising from high-excitation levels of H2 found in NGC 2023. This was the first image to reveal an H2 line in the optical CCD regime. It depicts molecular gas at the highest excitation level known (>44,000 K) above the ground state. The vertical scale here covers only 2.4 arcmin of the sky and the emissions here are so far into the red end of the spectrum as to be unobservable by humans. Be grateful you are not a graduate student who gets stuck with parsing out spectral lines in objects like these. OTOH, it’s an A in Spectroscopy 404b. Source: McCartney 1999 Fig.3.

A unique feature of NGC 2023 is a shell of neutral hydrogen (H2) surrounding HD 37903 out to a radius of about 0.65 light years. The shell amazingly emits light not by the photoionisation of hydrogen but by a unique process called vibrational fluorescence (1, 2, 3, 4). It is the first reflection nebula known to exhibit this type of emission. " of "24 21 110

How to know champagne flow when you see it: photo-ablation caused by NGC 6193 Ara

Source: Peter Ward, Barden Observatory, Australia

Multiple gas flow fields in M8 Lagoon Southern Wall

Bright edges are HII fluorescence, as described on p.21.

Champagne flow

4 stages of pillar erosion

Illuminating source of this image is NGC 6530 out of image at top. N6530 is a ~2 Myr cluster shedding its natal gas visible as red HII here. N6530 has 2 Otype & 21 B-type stars & lies in a larger region of “conveyer belt” sequential star formation from ongoing fresh gas infall, with 1523 X-ray emitting T-Tauri or proto stars hidden behind natal gas & dust. The wall of dense dust shown here lies closer to us & is not massive or cold enough to initiate star formation. The erosion surface seen here will continue abating surface gas through the HII fluorescence mechanism until N6530’s O and B giants go supernova.

The M8 “Lagoon” is a dense stream of gas & dust being transported toward bottom of image by magnetic flux tube confinement pressure. This portion is part of the side lobe of the Lagoon magnetic loop. Part of the dust stream crosses the Southern Wall ridge, partially eroding it as seen here.

Envoi

If the human eye can detect only one percent of whatâ&#x20AC;&#x2122;s there, what else are we missing?

Nightfall Vol. 3 No. 1 2019 copyright © 2019 by Astronomical Society of Southern Africa on behalf of the original authors, photographers, and illustrators whose work appears in this publication. Text and image copyrights © are reserved by the original producers.  Electronic editions are available free of charge from the Deep-Sky Observing Section of the Astronomical Society of Southern Africa, Observatory, Cape Town. Printed copies may be ordered from Atelier Books LLC, Postnet 18, P. Bag X1672, Grahamstown 6140, South Africa. E-mail: assa.nightfall@gmail.com and atelierbooks@gmail.com. Printed and bound in South Africa.

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